The transport characteristics of block and star PEG-PNIPAAm copolymer hydrogels were investigated. Using a side-by-side diffusion cell, the diffusive properties of rhodamine B, vitamin B12, and FITC-dextran were studied. Results verify that the hydrogels can be used as controlled release systems. PEG-PNIPAAm hydrogels were prepared from the polymerization of NIPAAm on different structure of PEG with molecular weight (MW) ranging from 2000 Da to 10 000 Da, and architecture of 2-arm and 4-arm. The effect of hydrogel concentration on the diffusion coefficients was studied by preparing 17%, 20% and 23% w/w hydrogels. The diffusion coefficients were found to vary exponentially with the hydration volume and the molecular weight of solutes, a result which is consistent with the free volume interpretation. An increase in PEG MW increased the diffusion coefficients, but the magnitude was smaller than that from varying the concentrations. Lastly, the diffusive characteristics for 2-arm and 4-arm PEG-PNIPAAm hydrogels were found to be similar.