We have developed a spectrometer for studying secondary‐electron yields from ion‐to‐electron converter dynodes. Ions, formed by surface ionization, are mass‐analyzed in a small double‐focusing mass spectrometer which is followed by a post‐accelerating lens to increase the kinetic energy of the ions to ∼30 keV before they strike an ion‐converter dynode. The ion‐converter dynode is a flat, narrow strip positioned along the the axis of rotational symmetry of a cylindrical lens and is rotatable about this axis. Secondary electrons are emitted from the thin‐film oxide surface of the converter dynode. They are accelerated to ∼30 keV in the cylindrical lens and are focused onto a silicon solid state detector for pulse‐height analysis of the secondary‐electron spectrum. Idividual secondary‐electron groups containing 1,2,3,…, etc., electrons are resolved and recorded in a 400‐channel pulse‐height analyzer. Deconvolution of the resulting energy spectrum gives precise estimates of secondary electron yields for monatomic or polyatomic ions of different mass, atomic number, energy, and angle of entry. Experimental results are given for thin films of oxidized aluminum, plasma‐anodized aluminum, and for an activated beryllium‐copper surface. Our results support the hypothesis that the Poisson distribution represents the minimum statistical variance (most sharply peaked distribution) that is physically attainable for secondary electron emission from an ion‐converter dynode.