Malaysia is amongst the world’s top producers of palm oil and the current planted area is around 4.5 million hectares. The palm oil industry generates vast amounts of palm biomass, especially empty fruit bunches (EFB) (from the mills), oil palm fronds (OPF) and oil palm trunks (OPT) (during routine pruning and from the field during replanting). Oil palm biomass can be used efficiently after further treatment, either by physical or chemical means. In this study, the swelling and dissolution mechanisms of the lignocellulosic biomass by ionic liquids were compared. There are five modes in describing the swelling and dissolution for cotton and wood cellulose fibre, and these were compared to the results obtained. Depending on the quality of the solvent, disintegration into rod-like fragments and ballooning, followed subsequently by dissolution were all observed among the oil palm fibre. In a typical dissolution trial, 5 wt % of oil palm biomass and cellulose fibre from EFB, OPF and OPT were treated with two different ionic liquids: 1-butyl-3-methylimidazolium chloride/dimethyl sulphoxide ([bmim]Cl)/DMSO and 1-ethyl-3-methylimidazolium chloride/dimethyl sulphoxide ([emim]Cl)/DMSO at a ratio of 80:20 wt %. They were heated at different fixed times, namely 4, 6, 8, 16 and 24 hr for untreated oil palm biomass, and 1, 2 and 3 hr for cellulose fibre. The mechanisms of swelling and dissolution were monitored by optical microscopy.