The optical gap of tetrahedrally coordinated hydrogenated amorphous semiconductors is defined in various ways in the literature. These differently defined optical gaps are the Tauc gap, the Cody gap, the Sokolov gap, and the two-isoabsorption gaps. In this work, these differently defined optical gaps are determined for a set of good quality a-Si1-xCx:H with x < 0.2 by measuring optical absorption coefficients. From these measurements empirical relations were derived between these differently defined optical gaps and carbon content (x) in the alloys. Variation of optical gaps with hydrogen content in the films is also discussed in correlation with structural disorder in the alloys. (C) 2003 Published by Elsevier Science B.V.