1-(p-Substituted benzyl)-2,4,6-triphenylpyridinium cations react with β -diketone anions by mechanisms which depend on the para-substituent. The p-methoxybenzyl derivative undergoes SN1 displacement yielding O- and C-benzylated products. The pnitrobenzyl compound reacts by a chain radicaloid mechanism and gives high yields of C-pnitrobenzylated diketones. The parent benzyl compound forms some C- and some O-benzylated products, together with bibenzyl, probably by a radical chain reaction which was suppressed by radical traps. © 1990.