A method for preparing a viscosity stable amido functional polysiloxane having a group .dbd.NCO(CHR).sub.n OH connected with a silicon atom of a siloxane unit of the polysiloxane comprises; i) preparing a viscosity stable intermediate amino functional polysiloxane having one or more siloxane units according to the general formula R.sup.2.sub.q' (HR.sup.4 N R").sub.r' SiO.sub.(4-(q'+r'))/2, any remaining units of the intermediate being of the general formula R.sup.2.sub.q SiO.sub.(4-q)/2, by condensation polymerisation; ii) adding an alcohol or substituted alcohol to the intermediate or to organo-silicon compounds used to produce the intermediate by condensation polymerisation, the alcohol or substituted alcohol being of the general formula R.sup.1 OH, wherein R.sup.1 is a substituted or unsubstituted alkyl group of from 4 to 30 carbon atoms, and; iii) reacting the viscosity stable intermediate polysiloxane resulting from ii) with a lactone of the general formula ##STR1## wherein R and n are as defined above. The amido functional polysiloxanes are useful as treating agents for fibrous materials, for example textile fabrics.