INTRODUCTION The deleterious effects of high-temperature@ high-pressure hydrogen environments upon the mechanical properties of steels have been recognized at least since the turn of the century. In 1960@ Class(1) reviewed the general problem of hydrogen damage and described the evolution of German steels intended for service in hydrogen. The steels he described contained one or more of the following alloying elements: chromium@ molybdenum@ vanadium@ tungsten@ nickel@ cobalt@ titanium@ columbium@ and tantalum. More recently@ in 1964@ Fletcher and Elsea(2) compiled a comprehensive review of the effects of hydrogen in steel. The Welding Research Council has also issued a Bulletin(3) which is an interpretive report on the effects of hydrogen in pressure-vessel steels. Hydrogen embrittlement of vessels and structures employed in the various hydrogenation processes was the principal problem first encountered when using hydrogen. This problem was largely overcome through the study of the heat treatment of the steels used in reactor vessel fabrication and by the addition of elements which lower the ductile-brittle transition temperature. With the development of hydrogenation processes requiring the use of higher pressures and temperatures@ the problem of deterioration of the mechanical properties of the steel by the process of decarburization plus internal fissuring@ known as hydrogen attack@ arose. It has been firmly established that hydrogen attack results from the reaction of hydrogen@ present at high pressures in the pre-existing microvoids in the steel@ with carbon from carbides in the steel to produce non-diffusing methane which expands the voids to fissures (4-11). The hydrogen attack reactionaccelerates the removal of carbon from the microstructure and causes the dissociation of the carbides. The primary guide in recent years to the selection of steels for service in hydrogen has been a chart first published by Nelson in 1951(12). An updated version of the chart can be found in API Publication 941(13). This chart is shown in Figure 1. The Nelson chart@ based on actual behavior of structural steels in commercial installations@ indicates recommended operating limits of temperature and hydrogen partial pressure for certain carbon and low-alloy steels. The principal aim of the present project has been to expand our ability to predict the hydrogen pressure and wall temperature conditions that will result in permanent hydrogen damage (fissuring) in low-alloy steels. Many investigators have attempted to study the hydrogen attack phenomenon. However@ most previous studies were conducted by hydrostatically exposing steel specimens to high gaseous hydrogen pressures and temperatures. This technique does not accurately simulate the exposure conditions encountered in pressure vessels. Thygeson(14) convincingly showed that the exposure technique has a very significant effect on the degree of hydrogen attack. He compared the room temperature tensile strengths of steel before and after exposure to hydrostatic high pressure hydrogen and pressure bomb test high pressure hydrogen. Therefore@ to obtain a better understanding of the hydrogen attack phenomenon in pressure vessels@ a qynamic exposure technique@ an experimental situation with a sustained hydrogen gradient@ must be used. Such a dynamic exposure technique was designed by Stueber(7@8) and subsequently used by Bisaro(9@10)