An evaporator assembly unit, especially for a vehicle heater or a reformer arrangement of a fuel cell system, includes a wall arrangement (12) enclosing an evaporation chamber (20) with a circumferential wall (14) and with a bottom wall (16). An air introduction shoulder (18) extending in the direction of a wall longitudinal axis (L) is provided with a plurality of first air introduction openings (22). Evaporator medium (26), that is porous at least in some areas, is provided on the side of the wall arrangement (12) facing the evaporation chamber (20). An auxiliary air opening arrangement (34) with at least one second air introduction opening (36) is provided in the wall arrangement (12).