The mixing required for post-mould gelatinisation of syntactic foams using starch as binder was studied. It was found that starch particles tend to adhere to hollow microspheres during mixing, forming agglomerates. A transition in the buoyancy of microsphere-starch phases was found at a particular volume fraction of starch. The transitional point was close to a volume fraction of starch at which a calculated relative density for a system consisting of multiple starch particles per microsphere was unity. Starch-microsphere inter-distance appears to be an important parameter affecting starch content in an agglomeration.