The lattice parameters of the body‐centered cubic phase (martensite) and the face‐centered cubic phase (austenite) were determined for a series of Fe&sngbnd;Ni alloys. Compositions ranged from pure Fe to Fe‐35 at.% Ni. Careful film and diffractometer Debye‐Scherrer techniques on powder and bulk samples and computerized calculations and error analyses were employed. Results indicate that the martensite lattice parameter reaches a maximum between 10 and 20 at.% Ni and the room‐temperature lattice parameter of the face‐centered cubic phase has a maximum in alloys having greater than 35 at.% Ni.