Sizing agents for carbon fibers

A sizing agent for carbon fibers consisting of bisphenol-polyalkylene glycol etherester copolymer shown by the following formula:

• ##STR8## where Y is ##STR9## the segments inside parentheses being a block or random copolyether, p being 0 or an integer 1-10, q being 0 or an integer 1-20, and (p+q) being equal to or greater than 1, Z is ##STR10## and A.sub.1 and A.sub.2 may be identical or different and are ##STR11## R.sub.1 and R.sub.2 being identical or different and H or CH.sub.3, X being ##STR12## the segments inside parentheses being a block or random copolyether, r and s being 0 or an integer 1-15, and (r+s) being equal to or smaller than 15 and equal to or greater than 1.