Various lignocellulosic substrates, including microcrystalline cellulose, pinewood and hemp fibres, were treated with triethylaluminium and triethylboron in suspensions, in non-swelling media. Chemical and spectroscopic analyses show that coupling surface reactions took place between one or two metal-to-carbon (Mt–C) bonds and the OH groups of the natural polymers. The remaining unreacted Mt–C bonds were exploited to graft numerous molecular structures derived from alcohols and amines. The occurrence of the second reaction was proved by Fourier-transform infrared spectroscopy, elemental analyses, X-ray photoelectron spectroscopy and contact angle measurements.