A method of deoxygenation of tall oil as well as methods for the production of aliphatic hydrocarbons and polymerizable monomers from tall oil. Sulphurous crude tall oil together with hydrogen gas is fed into a reactor comprising a catalyst bed. The oil is catalytically deoxygenated by hydrogen in the bed by use of a sulfided metal catalyst, e.g. a NiMoS catalyst. The flow exiting the reactor is cooled down and a hydrocarbon-bearing liquid phase is separated from a gas phase, followed by subjecting the liquid phase to distillation for removal of useless aromatic hydrocarbons and then to steam cracking to form a product containing olefins such as ethylene or propylene. By regulation of the deoxygenation temperature to be at least 270° C. but less than 360° C. the yield is rich in linear and cyclic aliphates that usefully turn to olefins in the steam cracking, while formation of napthalenes is reduced.