A process for preparing a Schiff base crosslinkable aqueous dispersion of a polyurethane A the process comprising the steps of: (a) reacting the following components one to four (two and four where present) to form an acidic isocyanate terminated prepolymer that comprises anionic or potentially anionic functional groups thereon; where: component one comprises 10 to 80% by weight of at least one polyisocyanate optionally containing at least one anionic or potentially anionic dispersing group; optional component two comprises up to 15% by weight of at least one isocyanate-reactive polyol containing at least one anionic or potentially anionic dispersing group; component three comprises 15 to 85% by weight of at least one isocyanate reactive polyol other than component two if present, and having a weight average molecular weight greater than or equal to 500 Daltons optionally containing at least one anionic or potentially anionic dispersing group; and optional component four comprises up to 20% by weight of at least one isocyanate reactive polyol other than component three and two if present and having a weight average molecular weight less than 500 Daltons; where if component two is not present component one or three comprise at least one anionic or potentially anionic dispersing group; where the amounts of components one to four are expressed as a weight percentage calculated from the total amount of the above components (i.e. one and three and optional two and/or four where present) being 100%; and where the mixture used in step (a) is substantially free of volatile amines and N-alkyl pyrrolidinones; (b) adding to the reaction mixture from step (a) an alkali metal neutralising agent in an amount from 0.05 to 6 parts by weight substantially to neutralise the isocyanate terminated prepolymer obtained from step (a); where the amount (in weight parts) of the alkali metal neutralising agent is calculated based on the weight of alkali metal in the neutralising agent relative to the total amount of components one to four in step (a) being equal to 100 parts; and (c) reacting the neutralised prepolymer from step (b) with an active hydrogen compound to extend the chain of the prepolymer to form an aqueous dispersion of polyurethane A. (a) reacting the following components one to four (two and four where present) to form an acidic isocyanate terminated prepolymer that comprises anionic or potentially anionic functional groups thereon; where: component one comprises 10 to 80% by weight of at least one polyisocyanate optionally containing at least one anionic or potentially anionic dispersing group; optional component two comprises up to 15% by weight of at least one isocyanate-reactive polyol containing at least one anionic or potentially anionic dispersing group; component three comprises 15 to 85% by weight of at least one isocyanate reactive polyol other than component two if present, and having a weight average molecular weight greater than or equal to 500 Daltons optionally containing at least one anionic or potentially anionic dispersing group; and optional component four comprises up to 20% by weight of at least one isocyanate reactive polyol other than component three and two if present and having a weight average molecular weight less than 500 Daltons; where if component two is not present component one or three comprise at least one anionic or potentially anionic dispersing group; where the amounts of components one to four are expressed as a weight percentage calculated from the total amount of the above components (i.e. one and three and optional two and/or four where present) being 100%; and where the mixture used in step (a) is substantially free of volatile amines and N-alkyl pyrrolidinones; component one comprises 10 to 80% by weight of at least one polyisocyanate optionally containing at least one anionic or potentially anionic dispersing group; optional component two comprises up to 15% by weight of at least one isocyanate-reactive polyol containing at least one anionic or potentially anionic dispersing group; component three comprises 15 to 85% by weight of at least one isocyanate reactive polyol other than component two if present, and having a weight average molecular weight greater than or equal to 500 Daltons optionally containing at least one anionic or potentially anionic dispersing group; and optional component four comprises up to 20% by weight of at least one isocyanate reactive polyol other than component three and two if present and having a weight average molecular weight less than 500 Daltons; (b) adding to the reaction mixture from step (a) an alkali metal neutralising agent in an amount from 0.05 to 6 parts by weight substantially to neutralise the isocyanate terminated prepolymer obtained from step (a); where the amount (in weight parts) of the alkali metal neutralising agent is calculated based on the weight of alkali metal in the neutralising agent relative to the total amount of components one to four in step (a) being equal to 100 parts; and (c) reacting the neutralised prepolymer from step (b) with an active hydrogen compound to extend the chain of the prepolymer to form an aqueous dispersion of polyurethane A.