The structure of a class of rate ½ convolutional codes called complementary codes is investigated. Of special interest are properties that permit a simplified evaluation of free distance. Methods for finding the codes with largest free distance in this class are obtained. A synthesis procedure and a search procedure that result in good codes up to constraint lengths of 24 are described. The free and minimum distances of the best complementary codes are compared with the best known bounds and with the distances of other known codes. © 1971, IEEE. All rights reserved.