A novel mass spectrometer with which masses of ions are determined in terms of high harmonics of their cyclotron frequencies is described from both experimental and theoretical viewpoints. Half‐width resolution is adjustable electrically over a range of values presently between 10 000 and 25 000 at all mass numbers below 250. By means of a highly precise technique of matching peaks, relative doublet separations in frequency (&Dgr;f/f) exactly equal to those in mass (&Dgr;M/M) are determined with great ease and accuracy on a ratio scaler. Masses have been determined with an accuracy of better than one part in 107, which is comparable with the best yielded by conventional instruments. Measurements should be of particular importance, even if of no greater precision than others, because the very different techniques employed make unlikely the occurrence of unsuspected systematic errors of the same nature as in other work. Sources of systematic error are discussed. It is believed that most of these have been eliminated, though avoidance of the not uncommon errors caused by differences in orbit distributions in doublets involving ions formed by dissociation has so far led to such large reductions of intensity as to restrict greatly the number of such important doublets for which reliable data have been obtainable. The synchrometer is apparently at a disadvantage to some deflection‐type spectrometers in the matter of intensity, to some extent in principle and to a greater extent (to date) in practice. This disadvantage is enhanced by the difficulties of using an electron multiplier for detecting ions in strong magnetic fields. It is believed that the latter difficulties are not insuperable and it seems reasonable to expect that further work on increasing the intensity and improving the focusing of ions from the source will greatly enhance the number of results obtained. The possibilities of using the instrument for analytical purposes are interesting but so far largely unexplored.