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Iron a l l o y s c o n t a i n i n g 20 and 30 pct Ni and 3 to 4 cu cm H p e r 100 g m e t a l have b e e n s u b j e c t e d to slow s t r a i n - r a t e t e n s i l e t e s t s in a study of hydrogen e m b r i t t l e m e n t . In the lower n i c k e l m a s s i v e m a r t e n s i t e alloy, e m b r i t t l e m e n t is m a n i f e s t as the c r a c k i n g of p r i o r a u s t e n i t e g r a i n b o u n d a r i e s and is s e v e r e at room t e m p e r a t u r e but l e s s m a r k e d at -196~ while in the higher nickel a c i c u l a r m a r t e n s i t e alloy, the e m b r i t t l e m e n t o b s e r v e d at 20~ does not o c c u r at -196~ Hydrogen e m b r i t t l e m e n t in these m a t e r i a l s is b e l i e v e d to be the r e s u l t of high h y d r o g e n contents in the v i c i n i t y of the p r i o r a u s t e n i t e g r a i n b o u n d a r i e s c o m b i n e d with s t r e s s c o n c e n t r a t i o n s caused by b o u n d a r y p e r t u r b a t i o n s which r e s u l t from the i m p a c t of the m a r t e n s i t e s h e a r s . D u r i n g d e f o r m a t i o n , m i c r o c r a c k s f o r m and propagate in the p r i o r a u s t e n i t e g r a i n b o u n d a r i e s , probably a s s i s t e d by i n t e r n a l hydrogen p r e s s u r e and the l o w e r i n g of c r a c k s u r f a c e e n e r g y by hydrogen a d s o r p t i o n . The t e m p e r a t u r e dependence and the effect of the type of m a r t e n s i t e on the e m b r i t t l e m e n t can be explained by t h e i r effects on the h y d r o g e n content and s t r e s s c o n c e n t r a t i o n s at p r i o r a u s t e n i t e g r a i n b o u n d a r i e s d u r i n g d e f o r m a t i o n . T H E p r e s e n c e of h y d r o g e n in bcc m a s s i v e and a c i c u l a r F e - N i m a r t e n s i t e s r e s u l t s in p r i o r a u s t e n i t e g r a i n b o u n d a r y c r a c k i n g when the m a t e r i a l s a r e s t r e s s e d . P r e v i o u s i n v e s t i g a t i o n s 1'2 of the effects of h y d r o g e n on t h e s e m a t e r i a l s have b e e n r e s t r i c t e d to d e l a y e d r u p t u r e t e s t s during or after e l e c t r o l y t i c hydrogen c h a r g i n g . Schuetz and R o b e r t s o n 1 showed that both m a s s i v e (10 pct Ni) and a c i c u l a r (30 pct Ni) m a r t e n s i t e s were s u s c e p t i b l e to hydrogen e m b r i t t l e m e n t at hydrogen cont e n t s of 3 and 30 cu cm H per 100 g m e t a l , r e s p e c t i v e l y . More r e c e n t l y Hyspecka et al. 2 r e p o r t e d two s t a g e s in the delayed r u p t u r e b e h a v i o r of a 22 pct Ni m a s s i v e m a r t e n s i t e ; the stages o c c u r r e d at different h y d r o g e n c o n t e n t s and were t e r m e d " v o l u m e " and " s u r f a c e " effects. The explanation offered for the f o r m e r effect was that e m b r i t t l e m e n t r e s u l t s from the f o r m a t i o n of m i c r o c r a c k s in the v i c i n i t y of p e r t u r b a t i o n s in the p r i o r a u s t e n i t e g r a i n b