New data including measured reflectance (%Ro), programmed open-system pyrolysis data, and kerogen elemental analyses obtained on the Mississippian Barnett Shale in the Fort Worth Basin, Texas, indicate that secondary-gas generation starts at 1.5% Ro and not at the previously prescribed 1.1% Ro. Oil-cracking kinetic parameters derived from pyrolysis experiments in the presence and absence of water indicate that secondary-gas generation will not occur at a thermal maturity as low as 1.1% Ro and requires a minimum thermal maturity of 1.5% Ro. This difference is especially important in using the Barnett Shale as an analog for evaluating other possible shale-gas plays. The new reflectance measurements have a good relationship with hydrogen indices (HIs) and compare well with other published data sets. However, the relationship does not compare well with the previously published data used to prescribe 1.1% Ro as the start of secondary-gas generation in the Barnett Shale. This discrepancy is attributed to differences in measured %Ro values and not attributed to differences in the HI values. Lack of publicly available information on the previously reported %Ro values makes it difficult to ascertain the reason for their lower values. These lower %Ro values also have impact on the previously prescribed relationship for estimating %Ro from the temperature at maximum yield by programmed open-system pyrolysis (Tmax). As a result, the new data do not agree with a previously described relationship, and the considerable scatter makes the new relationship unreliable. However, the relationship between the HI and %Ro has less scatter, which indicates that HI offers a better proxy in calculating %Ro than Tmax for the Barnett Shale. Comparison of various programmed open-system pyrolysis methods (i.e., Rock-Eval II, Rock-Eval 6, Source Rock Analyzer, and Hawk) indicates that variations in HI are within ±10% of one another. An HI of at least 44 mg/g total organic carbon is prescribed as a more certain limit for the start of secondary-gas generation and prospective in situ gas-shale accumulations.