A process for producing a substrate having a functionalised surface, which process comprises contacting the substrate with a carbene precursor, which carbene precursor is a compound of formula (III) or (IV) whose substituent groups are defined herein: (b) generating a carbene reactive intermediate from the carbene precursor so that it reacts with the substrate to functionalise the surface, thereby yielding an activated substrate; and (c) further functionalising the activated substrate obtained in (b). In (c), the activated substrate may be further functionalised by treating the activated substrate with a diazonium salt for the introduction of colour and/or another desired activity, and/or by treatment with hydrogen peroxide to produce a biocidal substrate. The invention further relates to carbene precursor compounds for use in the surface functionalisation process, and to processes for preparing certain precursor compounds.