Experimental studies on the production of hydrogen (H2) gas from catalytic co-gasification of mixtures of plastic high density polyethylene (HDPE) derived from municipal solid waste (MSW) and biomass rubber seed shell (RSS) are conducted in a non-isothermal thermogravimetric analysis (TGA) equipment coupled with mass spectrometer (MS). A commercial nickel is selected as the catalyst in this process. The main objective of the present study is to assess the combined effect of the operating parameters such as temperature, HDPE particle size, RSS particle size, and percentage of plastics in the mixtures on the response variable i.e. production of H2 from the system. The steam generated by the superheater at temperature of 110°C is injected at flowrate of 0.005mLmin-1 meanwhile argon gas is supplied at flowrate of 100mLmin-1 into the TGA-MS system. The steam to feedstock and catalyst to feedstock ratio of 1 and 0.1 are used respectively. A central composite design (CCD) based on response surface methodology (RSM) is used for the experimental design. The studies are carried out at temperature of 500-900°C, HDPE particle size range of 0.125-0.625mm, RSS particle size of 0.125-0.625mm and percentage of HDPE in the mixture of 10-40wt on the response variable of H2 production. The optimum process parameter for maximum H2 production in the system is determined. © Springer International Publishing Switzerland 2015.