In a series of pictures taken with a multiplate cloud chamber, we have observed a number of charged unstable particles considerably heavier than π-mesons, of which some decay at rest (S particles) and some decay in flight (V+/- particles). In each case the charged decay product is a particle of mesonic mass. From our results, which are not yet completely conclusive, it appears that most of the observed events can be explained by the two-body decay of a single kind of particle. The evidence concerning the mass of this particle, its mean life, and the nature of the neutral decay product is discussed. Our observations are compared with those of other experiments.