The propagation rate coeff. kp for the free-radical polymn. of 1,3-butadiene was detd. as a function of temp. in the range of 30-60 DegC in bulk and as a function of the concn. of 1,3-butadiene in chlorobenzene at 50 DegC using a combination of a UV flash-lamp induced polymn. and a measurement of the mol. wt. distribution of the produced polymer by size exclusion chromatog. Differences of up to around 45% resulted even for the same concns. and temps. when these measurements were compared with recent detns. of kp of 1,3-butadiene in chlorobenzene in which just such an exptl. setup was used. This finding was essentially attributed to large differences in the radical termination rates which prevailed in the reaction vols. under the current conditions of the pulse-induced polymn. expts. Moreover, in the vicinity of bulk concns. of 1,3-butadiene in chlorobenzene, kp apparently decreases with concn.