High pressure gaseous oxygen is obtained safely and without compression by heating pumped liquid oxygen in a printed circuit type heat exchanger having layers of transversely extending laterally spaced channels with each layer being in thermal contact with at least one other layer. Oxygen is vaporized in channels of oxygen-layers against heat exchange fluid passing through channels of heat exchange layers. The walls of the oxygen layer channels are formed of ferrous alloy and have a cross-section, in a plane perpendicular to the direction of flow, having a thickness at its narrowest of at least about 10%, and on average at least about 15%, of the combined hydraulic mean diameters of the adjacent channels, and the ratio of cross-sectional area, in said plane, of the walls to the cross-sectional area of the channels is no less than about 0.7.