A basic study on the catalytic gasification of polyolefins such as PE and PP, which account for a major part of general waste plastics, was conducted in order to develop a technique for effective recycling of these wastes. In the case of PE, the gasification of PE is considered to consist of the following scheme: polymer → catalytically degraded polymer → catalytically degraded oligomer → liquid component → gas component. The gasification of PE does not occur directly from the polymer chains, but gaseous C4 substances are selectively found from the liquid components with the highest branching frequency. The overall yield of C4 components including isobutane was 74.5 and 60.5% molar for PE and PP. These liquid components (gasification precursors) have the branching frequencies. For example, a molecule with M̄w of 400 contains about eight branches for every 30 methylenes. From the catalytic gasification of PE, PP, and PIB, the gas conversion rate is also found to increase with increasing frequency of the backbone branching. It is concluded that the branching frequency is the key factor governing the gas conversion rate of polyolefins.