The thermal degradation behavior at 350-degrees-C of poly(hydroxybutyric acid), "Biopol", has been studied by pyrolysis-GC using a microthermocouple-controlled filament. The overall pyrolysis mechanism-an array of parallel and consecutive processes-has been elucidated by a novel method. This involves the measurement of product yields as a function of sample thickness, for samples in the microgram range, pyrolyzed for a chosen duration (5 s was used in the present study). This approach provides indirect control of the residence time of primary products in the melt and thereby facilitates the detection of secondary reactions. From quantitative measurements of bonus yields and deficit yields, it is shown, for example, that only trans isomers are formed as primary products; trans-cis isomerisations then occur as secondary reactions. Moreover, although monomeric, dimeric, and trimeric products are formed by primary processes, there is evidence that a trimeric product is also formed by a consecutive reaction mechanism. Somewhat unexpectedly, the tetrameric products are formed exclusively by secondary reactions. This observation casts some doubt on the view that the pyrolysis products from this polymer can be accounted for entirely in terms of random (beta-elimination) scissions.