The compositions of optically homogeneous titanomagnetites and ferrian ilmenites in basalts were determined by electron probe microanalysis. The objectives were (1) to obtain accurate compositional distributions for the two types of grain in a type of rock widely used for paleomagnetic purposes and (2) to interpret these distributions in terms of the magnetic properties of the rocks. Fe∶Ti ratios by weight were obtained from 96 optically homogeneous titanomagnetite grains in 47 specimens (20 normal, 27 reversed) from three suites of basalts (Icelandic lava flows, Scottish dikes, and Columbia plateau, Oregon, lava flows). Curie points deduced from these ratios assuming stoichiometric proportion were compared with the measured Curie points. In general, the measured Curie points were much higher than the predicted values, and this difference was considered to be due to slight oxidation of the titanomagnetites. The suggested mechanisms of Verhoogen for the self-reversal of titanomagnetites by ionic ordering and oxidation were examined in the light of the probe data, from which it was shown that the 27 reversed specimens were unlikely to be self-reversed by any of these mechanisms. Fe∶Ti ratios by weight were also obtained from 47 optically homogeneous ferrian ilmenite grains in 26 specimens (6 normal, 20 reversed) from Iceland, Scotland, Ireland, South America, Oregon, and India. The compositions of all specimens lie in the region of the Carmichael ilmenite-hematite solid solution solvus curve in which stable solid solutions are allowed at room temperature but did not lie in any known region of self-reversal for the ilmenite-hematite series. It was concluded that the large proportion of reversed specimens was not caused by self-reversal in the α phase and that, in any case, the α phases examined could not have contributed appreciably to the remanence at room temperature.