Recent papers by Takenaka, Johnson and Kamide (Polymer 1994, 35, 3899, 3908) include some claims about the efficiency of azobisisobutyronitrile as an initiator of certain radical polymerizations. It was suggested that the efficiency factor is unity up to high conversions and then decreases quite markedly and abruptly. It is now shown that the treatment is probably in error, and that the conclusions may be untenable.