A combination of quantitative gas chromatography (for the lighter products up to Clo and for the determination of the chain-growth probability) and calculations based on a modified Anderson-Schulz-Flory (ASF) model (for the heavy products) provided instantaneous product determinations for studying the influence of feed composition, pressure, space velocity, and temperature on the rate and chain-growth probability of the reaction in a plug flow reactor. Material balances were often less than 100%, due to underestimation of the heavy products by the model. Inert species were shown to impede both the rate and the chain growth. Small pressure increases in the near-atmospheric region resulted in significant increases of the rate and the chain growth. Increasing space velocity resulted in a higher rate, but the chain growth was hardly affected. Over a wide conversion range, the rate was found to be a linear function of the temperature. These and other results attest to a multiplicity of the chain-growth probability and to diffusional limitations of the rates. The factors defining the chain-growth probability are discussed.