We present a new gas-grain chemical model that allows the grain-surface formation of saturated, complex, organic species from their constituent functional-groups–basic building blocks that derive from the cosmic ray-induced photodissociation of the granular ice mantles. The surface mobility of the funtional-group radicals is crucial to the reactions, and much of the formation of complex molecules occurs at the intermediate temperatures (~20–40 K) attained during the warm-up of the hot core. Our model traces the evolution of a large range of detected, and as yet un-detected, complex molecules.