The effects of hydrogen on the fracture behavior of manual SMA welds in carbon steel plate was investigated utilizing modified ½T compact tension specimens. Tests performed on these specimens in the presence of hydrogen were compared to similar tests in helium. These tests showed that hydrogen lowers J_C in both the heat affected zone and the base metal. In 350 psi helium, the experimental value of J_C in the heat affected zone (2826 in.-lbs./in.²) was greater than that obtained in the base metal (1650 in.-lbs./in.²). The tests conducted in 350 psi hydrogen resulted in a reduction in J_C for both the heat affected zone (1425 in.-lbs./in.²) and the base metal (59 in.-lbs./in.²). Furthermore, when compared to specimens tested in helium, it was determined that the material tearing modulus for specimens tested in hydrogen was significantly reduced. Slow stable crack growth occurred in all helium tests and in tests performed on the heat affected zone in hydrogen. However, unstable crack growth (fast fracture) was obtained for base metal tests in hydrogen. Fractographic studies revealed that the mechanism for all slow stable crack growth was microvoid coalescence; whereas, the surface of base metal specimens tested in hydrogen showed that fast fracture occurred by cleavage. Optical microscopy revealed that the fracture path for all base metal tests remained in the base metal, but that the fracture path for all heat affected zone tests moved towards the base. These observations, in conjunction with microhardness readings and quantitative metallography, were used to develop explanations for the observed behavior. These explanations include the combined effects of hydrogen, weld defects, residual stresses, grain size, and test variables such as temperature and specimen size and geometry. / M. S.