We studied the phase-transfer catalytic activity of hexaalkylguanidium salts, the final products of guanidine alkylation. We examined the catalytic activity of 12 kinds of guanidine salt in reactions starting in an organic phase or in an aqueous phase. The comparative control was tetrabutylammonium iodide (TBAI), a quaternary ammonium salt. In reactions starting in an organic phase, catalytic activity was mostly in the order of guanidium salts >= TBAI, indicating that more lipophilic salts have higher catalytic activities among the guanidium salts. In contrast, in reactions starting in an aqueous phase, catalytic activity was mostly in the order of TBAI>guanidium salts; more lipophilic salts had higher catalytic activity among the guanidium salts. However, in all the reactions, hexabenzylguanidium salts showed little catalytic activity. We found the cause to be the hydrolysis of hexabenzylguanidium salts by a strong base under conditions for phase-transfer reaction. In addition, we considered the mechanism of the catalytic activity of guanidium salts in relation to their structures.