Polarograms and cyclic voltammograms exhibit two waves for reduction of phenylpropadiene at a mercury electrode in dimethylformamide containing tetra-n-butylammonium perchlorate, the f i t wave signals reduction of phenylpropadiene to 1-phenyl-1-propene and the second wave is attributable to reduction of 1-phenyl-1-propene to 1-phenylpropane. However, the fmt wave is abnormally small because phenylpropadiene undergoes substantial rearrangement to 1-phenyl-1-propyne, which is reducible to 1-phenylpropane at nearly the same potential as 1-phenyl-1-propene. Controlled-potential electrolyses of phenylpropadiene, 1-phenyl-1-propene, and 1-phenyl-1-propyne at mercury pool cathodes have verified the processes elucidated by means of polarography and cyclic voltammetry. In the presence of diethyl malonate (a proton donor), base-catalyzed isomerization of phenylpropadiene to 1-phenyl-1-propyne is blocked totally; at a potential corresponding to the first wave for reduction of phenylpropadiene, electrolysis products are trans-1-phenyl-1-propene, cis-1-phenyl-1-propene, and l-phenyl-2-propene, whereas 1-phenylpropane and 1-phenyl-2-propene are obtained at a potential on the second reduction wave for phenylpropadiene.