Abstract A model insoluble oxygen-free phenanthrene—formaldehyde resin has been reductively alkylated by means of potassium—naphthalene in THF and subsequent treatment with alkyl halide. The extractability of the resultant alkylated products by benzene reaches 50% and depends on the size of the groups introduced. The polydispersity of the benzene-soluble portions and the presence of methyl groups in them indicate that decomposition processes are taking place at the methylene bridges as well. Mainly on the basis of n.m.r. analysis of the benzene-soluble fraction of a reductively trimethylsilylated resin, quantitative information about the silylation and decomposition processes was derived. The results obtained are of interest to the study of reductive alkylation of coals.