Gelatin prepared from animal sources is widely used as a stabilizer in vaccine formulations. The disadvantages associated with their use such as heterogeneity and allerginicity, have led to the development of recombinant human gelatins (rhGs) as a substitute. This study focuses on characterizing the structure and monitoring the physical stability of four molecular weights ($8.5, 25, 50, and 100 kDa) of rhGs as a function of temperature and pH. The information supplied should be useful in predicting the behavior of rhGs under formulation conditions. A number of spectroscopic techniques were employed in this study. Experimental results indicated that the solution properties of all four rhGs were unpredictable with micro-aggregation observed at various pH values. The 8.5 kDa rhG was found to be in a micro-aggregated state at pH 5 while the 25 kDa rhG was found to be more aggregated at pHs 5, 7, and 8. The properties of these aggregates have been analyzed as a function of temperature. The rhGs were also found to complex with the polyanion heparin through electrostatic and nonelectrostatic interactions. The stability of these complexes has been studied as a function of temperature and pH.