The invention relates to a method for pretreatment of a solid catalyst that is obtained by a reaction between an inorganic binder and phosphoric acid, whereby said method comprises a stage in which said catalyst is brought into contact with a mixture of hydrocarbons under swelling conditions of the catalyst up to a predetermined volume increase, with no significant volume constraint on said catalyst. The invention also relates to an operating process of said catalyst in an alkylation or oligomerization reactor.