Products obtained in the pyrolysis of high-density polyethylene (HDPE) can be very useful in plastic waste energy recovery or as raw materials in several processes in the chemical industry. The products obtained, however, depend on many variables such as temperature, volatile residence time, type of catalyst-polymer contact, or type of pyrolysis. Because of this, it is necessary to know how these factors can modify the yield and the composition of the volatile compounds obtained during the pyrolysis of different polymers. In this work, thermal and catalytic (HZSM-5) flash pyrolysis of HDPE at different residence times is widely studied using two different types of reactors: a pyroprobe apparatus, where the volatile residence time is in the range of few milliseconds, and a fluidized bed reactor, where the secondary reactions take place in a larger extension. Significant differences between thermal and catalytic pyrolysis were observed, and the influence of the residence time on the yield of the different compounds obtained, in the presence and absence of a catalyst, is discussed. Results show the equalizing effect of the secondary cracking reactions on the final yields obtained in thermal and catalytic conditions. The compounds generated can be grouped according to three types of behavior: compounds whose yields increase with residence time, compounds which are cracked by secondary reactions, and compounds which show a maximum in their production. The presence of the catalyst modifies the behavior of each product. The results obtained can only be interpreted as a contribution of the many different effects present, i.e., the presence or absence of the catalyst, the residence time of the volatiles, the type of solid-volatiles evolved-catalyst contact, and the effect of the catalyst on the primary and secondary reactions. The operating conditions selected simulate the usual industrial reactors in order to obtain significant conclusions useful for the knowledge of the adequate performance of the process on a large scale.