Methanol is reacted with branched monoolefins such as isobutylene, or mixtures of hydrocarbons such as mixed C-4's from or in a refinery, in the presence of methanesulfonic acid. A column (10) operation is described wherein methanesulfonic acid (11) and methanol (12) are fed in countercurrent fashion to hydrocarbons (13). Crude product ether is removed as a liquid sidestream (25) above the feed of methanesulfonic acid (11), while hydrocarbons depleted of the reactive branched monoolefins (24) are removed overhead. The methanesulfonic acid suppresses the vapor pressure of methanol relative to product ether.