Evolution rates of low-molecular-weight gas products (H-2, CH4, CO, and CO2) in pyrolysis and steam gasification of biomass (cellulose and lignin) were studied using thermogravimetric-mass spectrometric (TG-MS) analysis. Total gas yields were measured with a TCD-micro gas chromatograph (micro GC) at heating rates of 1, 10, and 100 K s(-1). Steam gasification of biomass demonstrated heating rate effects on gas evolution: 81 wt % of cellulose was converted into tar in pyrolysis; also, slow heating and steam gasification of nascent char occurred above 700 K, evolving H-2. Rapid heating significantly enhanced secondary pyrolysis of cellulose tar to yield H-2, CO, and CH4. In contrast, char formation at 500-773 K was dominant in lignin pyrolysis. Evolution of H-2 and CO2 were significantly increased by steam gasification of char. Insignificant influence of heating rate on carbon and hydrogen yield of lignin was observed. Over 70% of lignin's chemical energy was converted into gas, especially hydrogen.