The spin polarization P of the low energy cascade electrons excited with a primary unpolarized electron beam is measured with ultrathin films of permalloy (Ni80Fe20) as a function of film thickness, external magnetic field, and temperature T. Surface adsorbates of small concentrations of less than 10% of a monolayer can change the Curie point and the saturation value of P0(T→0) by as much as 30%. The Ta-substrate induces a magnetically dead region in permalloy. Conventional spin wave theory cannot account for the observed small T-dependence of the magnetization M. Films on a nonmagnetic substrate are compared to similar films coupled to bulk permalloy over an interface of Ta. The T-dependence of M with the coupled films can be explained by spin wave theory. At low T, the films coupled to the bulk exhibit a faster decrease of M than the uncoupled films. We propose that this thermal stabilization of the magnetization in very thin ferromagnetic films is due to quenching of the long wavelength spin modes. © 1989 Springer-Verlag.