A phenol-formaldehyde (PF) resite resin and phenol:2,6-di-t-butylphenol-formaldehyde (DTBPF) co-resite were reacted under conditions of microwave heating with N-(trimethylsilyl)imidazole (TMSI). Low degrees of reaction were observed for the PF resin (19%) and for the DTBPF co-resite (0.3-0.4%) as examples of macromolecular systems with very limited porosity. C-13 SPE MAS n.m.r. was an effective method to determine the degree of silylation and C-13 CP MAS n.m.r. gave very similar results. When a different reagent mix (1:1:1 N,Obis(trimethylsilyl)acetamide: N(trimethylsilyl)imidazole:trimethylchlorosilane) was used both C-13 and Si-29 SPE n.m.r. were in agreement that 100% silylation of the OH groups had been achieved for the PF resin. Thus, with proper choice of conditions and reagents, microwave-driven silylation is a very effective method of OH analysis even for systems of significantly more limited porosity compared with coals. (C) 1998 Elsevier Science Ltd. All rights reserved.