An aqueous crosslinkable coating composition suitable for the provision of thick coatings without incurring carbon dioxide bubble formation comprising: (A) an aqueous dispersion of at least one hydroxyl- and acid-functional vinyl polymer having a hydroxyl value of 8 to 250 mg KOH/g of polymer and an acid value of 2 to 55 mg KOH/g of polymer, where the contribution of at least 30% of the acid value results from a monocarboxylic acid(s) of formula: CH.sub.2 --CR.sup.1 --Q--CO.sub.2 H (1) where R.sup.1 is H or lower alkyl of 1 to 5 carbon atoms and Q is a spacer group of formula: [--C(.dbd.O)--X--(CH.sub.2).sub.m --].sub.k (2) where X is --O-- or --NH--, m is at least 1, preferably 2 or 3, and k is at least 1, preferably 2, 3, 4 or 5; and (B) at least one organic polyisocyanate. The acid of formula (1) preferably comprises .beta.-carboxyethyl acrylate.