While the broadness of the pyrolysis profile of most kerogens is described well by a parallel reaction model, the pyrolysis profile at a constant heating rate for certain well-preserved algal kerogens is narrower than can be described by a single first-order reaction. Further, these kerogens show an acceleratory period under isothermal conditions that is inconsistent with any parallel or nth-order reaction model. Three different models (serial, Bouster, and three-parameter) are tested against isothermal and nonisothermal pyrolysis data for a few samples, with the conclusion that the three-parameter model fits well and is the most stable and reliable. The three-parameter model reduces to a first-order model when the acceleration parameter is zero. The overall activation energy and frequency factor from this model are very close to those of the T-max-shift method recommended earlier.