Natural cellulose-based fibers, such as cotton, have been investigated as oil sorbent to remediate and recover oil spills. The unmodified fibers are hydrophilic and have a high capacity to absorb water. To circumvent this drawback, the hydrophobicity of fibers is usually enhanced by chemical modification using solution-based processes that produce significant amounts of chemical waste. In the present study, gas–solid solvent-free silylation reaction was used to graft alkyl groups on cotton fibers. The modified cotton fibers were characterized by IR-spectroscopy, TGA analysis and SEM–EDS. The degree of substitution varied between 0.1 and 0.3 per glucose residue. The ability of the fibers to remove oil from the surface of simulated sea-water was investigated and all the modified fibers have adsorption capacity at least five times that of unmodified cotton. To optimize the adsorption capacity, factors that affect oil adsorption were investigated, namely the effect of fiber–oil contact time, temperature, and length of the grafted alkyl chains. Cotton fibers grafted with dialkyl substituted silyl ethers, through solvent-free silylation reaction, were found to have the best adsorption capacity of ~ 18 g oil/g of modified cotton with a fiber–oil contact time of 10 min at 25 °C.