To provide a curing agent composition containing an active methylene type blocked polyisocyanate having a high heat stability and an excellent hue, a process for producing the same and a one-package thermosetting composition excellent in low-temperature curability and storage stability.

This invention provides an active methylene type blocked polyisocyanate composition having a pH of 8.5 or below and obtained by reacting an active methylene compound such as diethyl malonate with a polyisocyanate in the presence of a catalyst being a basic compound (e.g. sodium methylate) and neutralizing at least part of the catalyst with an acidic compound (e.g. dibutyl phosphate) and a one-package thermosetting composition containing the above composition and a polyhydroxy compound.