An investigation was conducted on 0.070-in.-thick 18Ni-9Co-5Mo, 300 ksi, maraging steel sheet to determine its applicability for use in high-strength pressure vessels. This study includes the effects of aging as-received material, re-annealing material, the combined effects of annealing and aging, and the influence of cold working and aging upon the properties and micro-structure of this maraging material. Austenite retained at room temperature was found as a result of aging at 950 F for 8 hr, or annealing in the temperature range of 1200 to 1250 F. Material containing significant amounts of austenite at room temperature was found to age unsatisfactorily; however, if material containing austenite is plastically strained, the aging response markedly improves. Material containing no austenite was also strengthened by virtue of the cold work, but a loss in strength was obtained after stretching beyond 2 per cent tensile plastic strain. Duplex aging cycles were investigated, and it was found that the heat treatment time required to reach a specified strength can be reduced by using a duplex aging cycle instead of a single aging cycle. In addition, the microstructures produced by various heat treatments are presented and interpreted in terms of the austenite-to-martensite transformation, martensite-to-austenite reversion, and precipitation reactions.