A waste heat recovery system (100) for an engine (101) comprises a fluid supply (104); one or more evaporators (120, 121) adapted to transfer waste heat from the engine (101) to fluid from the fluid supply (104) to heat the fluid to a superheated vapor; a condenser (134) having a condenser inlet (134′) in fluid communication with the one or more evaporators; and a pressure regulator (200) configured to limit the vapor pressure at the condenser inlet (134′).