A series of dideuterated primary phenylalkyl chlorides 1-chloro-3-phenylpropane-2,2-d2(4), 1-chloro-3-phenylpropane-1,1-d2(9), 1-chloro-4-phenylbutane-2,2-d2(8), 1-chloro-5-phenylpentane-4,4-d2(5), 1-chloro-5-phenylpentane-3,3-d2(6), and 1-chloro-5-phenylpentane-2,2-d2(7) were synthesized by alkylations of malonic ester with phenylalkyl chlorides. Deuterium was introduced by exchange of the sodium salt of the alkylated malonic ester with D2O, followed by hydrolysis and decarboxylation. The resulting acids were reduced by hydride to primary alcohols and these were converted to the dideuterated primary chlorides by decomposition of the intermediate chlorosulfite esters. Nuclear magnetic resonance (n.m.r.) spectra confirmed the position of the deuterium in the chlorides. Friedel–Crafts cyclialkylation of the deuterated phenylpentyl chlorides yielded labelled 1-methyltetralins in which the label was located by n.m.r. and mass spectroscopy. The results demonstrated that rearrangement precedes cyclialkylation of 1-chloro-5-phenylpentane. Extensive deuterium scrambling between C-1, C-2, and C-3 accompanied the cyclialkylation of5but not that of8. Reaction mechanisms are suggested to account for these results.