[en] The stopping power and straggling of alpha particles initially at 5.486 MeV have been studied in both solids and gases considered to be tissue equivalent with respect to their mean ionisation excitation potential. Measurements were made with a silicon surface barrier detector and the observed energy straggling compared with the theoretical predictions of Tschalar. The solids used were examined for uniformity of thickness by interferometry and it is concluded that the increased straggling observed for alphas travelling through solids as compared to those slowed down in gases is due to non-uniformities in thickness. The stopping powers were derived from polynomials fitted to the residual energy vs path length data. (Auth.)