The experimental behavior of isotropic ferrimagnetic insulators in second harmonic generation at microwave frequencies has been investigated and compared with the behavior predicted by the equations of motion with Landau‐Lifshitz and with Bloch‐Bloem‐bergen damping. In general, a close agreement between experiment and theory was found. The experiments were carried out with small samples of single‐crystal manganese ferrite, YIG, and other ferrimagnetic materials. Since harmonic generation results from asymmetries in the precessional orbit, measurements were made with disk‐shaped specimens in which the demagnetizing fields create the asymmetry. The technique employed a cavity simultaneously resonant to 8.5 and 17 kMc. Absolute values of field intensities and magnetizations were computed from absolute power measurements. A figure of meritFfor a ferrimagnet is defined. In a practical 2&ohgr; generator the output powerP2&ohgr; is expected to vary as the productF2Pi2, wherePiis the input &ohgr; power. In comparing materials at ferrimagnetic resonance (FMR),Fshould vary as &khgr;0″. This was verified experimentally, even at high drive levels. At constant frequency and drive levelFshould be nearly independent ofHdc. Only slight deviations from this were found at low drive levels.