diff --git a/src/datasets.jl b/src/datasets.jl
index ce50cfa2..a71bd01b 100644
--- a/src/datasets.jl
+++ b/src/datasets.jl
@@ -367,7 +367,7 @@ end
     seek(io, header_offset)
     f.end_of_data = header_offset + fullsz
 
-    track!(wsession, data, header_offset)
+    track!(wsession, data, h5offset(f, header_offset))
 
     cio = begin_checksum_write(io, fullsz - 4)
     jlwrite(cio, ObjectStart(size_flag(psz)))
@@ -377,7 +377,7 @@ end
     for attr in dataspace.attributes
         write_header_message(cio, f, attr)
     end
-    write_header_message(cio, Val(HmDatatype), 1 | (2*isa(dt, CommittedDatatype)); dt)
+    write_header_message(cio, Val(HmDatatype), 1 | (2*isa(datatype, CommittedDatatype)); dt=datatype)
 
     # Data storage layout
     if layout_class == LcCompact
diff --git a/src/explicit_datasets.jl b/src/explicit_datasets.jl
index 7168f70e..8026953b 100644
--- a/src/explicit_datasets.jl
+++ b/src/explicit_datasets.jl
@@ -144,12 +144,17 @@ function write_dataset(dataset::Dataset, data, wsession::JLDWriteSession=JLDWrit
     end
     offset = write_dataset(f, dataspace, datatype, odr, data, wsession, compressor)
     !isempty(dataset.name) && (dataset.parent[dataset.name] = offset)
-
     # Attributes
-    # TODO: this can be optimized by writing all attributes at once
-    for (name, attr) in pairs(dataset.attributes)
-        add_attribute(dataset, name, value, wsession)
+    attrs = map(collect(keys(pairs(dataset.attributes)))) do name
+       WrittenAttribute(f, name, dataset.attributes[name]) 
     end
+    dataset = get_dataset(f, offset, dataset.parent, dataset.name)
+    dataset.header_chunk_info = 
+        attach_message(f, dataset.offset, attrs, wsession;
+            chunk_start=dataset.header_chunk_info[1],
+            chunk_end=dataset.header_chunk_info[2],
+            next_msg_offset=dataset.header_chunk_info[3],
+            )
 
     return offset
 end
@@ -341,9 +346,9 @@ function add_attribute(dset::Dataset, name::String, data, wsession=JLDWriteSessi
     f = dset.parent.f
     prewrite(f) # assert writability
 
-    for attr in dset.attributes
-        if (attr isa ReadAttribute && attr.name == name) || (attr isa Pair && attr.first == name)
-            throw(ArgumentError("Attribute $name already exists. Attribute names must be unique."))
+    for attrname in keys(dset.attributes)
+        if name == attrname
+            throw(ArgumentError("Attribute \"$name\" already exists. Attribute names must be unique."))
         end
     end
     dset.attributes[name] = data
@@ -484,7 +489,7 @@ function allocate_early(dset::Dataset, T::DataType)
     for attr in dataspace.attributes
         write_header_message(cio, f, attr)
     end
-    write_header_message(cio, Val(HmDatatype), 1 | (2*isa(dt, CommittedDatatype)); dt)
+    write_header_message(cio, Val(HmDatatype), 1 | (2*isa(datatype, CommittedDatatype)); dt=datatype)
     for a in attributes
         write_header_message(cio, f, a, wsession)
     end
diff --git a/src/types.jl b/src/types.jl
index aa98ed54..c9e7ad77 100644
--- a/src/types.jl
+++ b/src/types.jl
@@ -114,17 +114,14 @@ referenced multiple times are written multiple times.
 """
 struct JLDWriteSession{T<:Union{Dict{UInt,RelOffset},Union{}}}
     h5offset::T
-    objects::Vector{Any}
-
     JLDWriteSession{T}() where T = new()
-    JLDWriteSession{T}(h5offset, objects) where T = new(h5offset, objects)
+    JLDWriteSession{T}(h5offset, objects) where T = new(h5offset)
 end
 JLDWriteSession() = JLDWriteSession{Dict{UInt,RelOffset}}(Dict{UInt,RelOffset}(), Any[])
-track!(s::JLDWriteSession{Union{}}, args...) = nothing
+track!(::JLDWriteSession{Union{}}, args...) = nothing
 function track!(s::JLDWriteSession, data, offset::RelOffset)
     if ismutabletype(typeof(data))
-        wsession.h5offset[objectid(data)] = h5offset(f, header_offset)
-        push!(wsession.objects, data)
+        s.h5offset[objectid(data)] = offset
     end
     nothing
 end