Photoresist pattern trimming compositions comprise: a polymer comprising as polymerized units a monomer comprising an acid-decomposable group, the decomposition of which group forms a carboxylic acid group on the polymer; a non-polymeric acid or a non-polymeric thermal acid generator; and an organic-based solvent system comprising one or more organic solvents. Methods of trimming photoresist patterns involve applying such pattern trimming compositions to a photoresist pattern that is formed from a photoresist composition comprising a photoacid generator and a polymer comprising acid-decomposable groups. The photoresist pattern trimming compositions and pattern formation methods find particular use in the formation of fine lithographic patterns in the semiconductor manufacturing industry.