The thermal expansion of 15 alkali halides has been investigated from 70 to 570 K. The single‐crystal specimens of LiF, LiCl, LiBr, NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, RbI, CsCl, CsBr, and CsI are examined using a high‐precision differential dilatometer in conjunction with an electronic micrometer. The results are compared with those available in the literature and are found to be in reasonable agreement. The Gru¨neisen parameter &ggr; has been calculated using the present thermal expansion data. For a given cation group, the value of &ggr; is very close, but increase in the cation size decreases &ggr; slightly. The indications are that in the temperature range under investigation &ggr; increases very slowly with temperature.