Preparation of polymer whiskers composed of p-oxybenzoyl (O) units and p-mercaptobenzoyl (S) units having graded composition was examined by the polymerization of S-acetyl-4-mercaptobenzoic acid (AMBA) and 4-(4-acetoxybenzoyloxy)benzoic acid (OO) or 4-[4-(4-acetoxybenzoyloxy)benzoyloxy]benzoic acid (OOO). Polymerizations were carried out in liquid paraffin at 300 degreesC for 6 h. The polymerization concentration was 1.0% and the molar ratio of O units in feed was 50%. Polymerization of AMBA and OO yielded copolymer whiskers by means of the crystallization of oligomers, which were 15 mum in average length and 0.4 mum in average width. The length and the width increased with polymer yield but the length increased preferentially rather than the width. The number of whiskers was constant during polymerization. The content of O units in the whiskers (chi(O)) prepared for 8 min was 62 mol % and then decreased to 49 mol % after 6 h. This fact revealed that the center part of the whisker was rich in O units, and the outer and tip parts were rich in S units. This whisker possessed a graded composition from O units to S units along the directions of the length and width. The oligomers rich in O unit sequences were preferentially precipitated in the early stage of polymerization, and afterward the oligomers poor in O units were precipitated to form the whiskers leading to a graded composition. Polymerization of AMBA and OOO yielded copolymer whiskers with a larger gradient composition than that of whiskers prepared from AMBA and OO.