For sensitive studies of molecular ions in electrostatic storage rings, the exact knowledge of the isobaric composition of stored beams from a variety of ion sources is essential. Conventional mass-filtering techniques are often inefficient to resolve the beam components. Here, we report the first isochronous mass spectrometry in an electrostatic storage ring, which offers a high mass resolution of Δm/m < 1 × 10-5 even for heavy molecular species with m > 100 u and uncooled ion beams. Mass contaminations can be resolved and identified at relative fractions down to 0.02%.