Unlabeled 1-vinylnaphthalene (1) as well as positionally D-labeled 1 have been prepared and subjected to gas phase pyrolysis at low partial pressures of the educt compounds in nitrogen. At 700°C, 1 rearranges exclusively to acenaphthene (2). Under the conditions applied a significant H-D-exchange in the 1-vinylnaphthalene as well as in the formed acenaphthene takes place. The results obtained in this way and those resulting from pyrolysis in hydrogen (instead of nitrogen) show that despite the very high degree of dilution the high selectivity of cycloisomerization from 1 to 2 is firstly controlled by H-atom driven radical chain processes, in which the cyclization of the 2-naphthylethylradical dominates the reaction course.