The paper presents a brief outline of a method for studying temporal and spectral characteristics of soft X-ray detectors in the photon energy range from several tens to a thousand of electron-volts. The measured characteristics (signal rise time, temporal resolution, absolute spectral sensitivity) are given as well as parameters calculated from the measured data (thicknesses of contact, "dead", and sensitive layers) of some types of fast silicon p-i-n photodiodes from different manufacturers (Siemens; Hamamatsu; Motorola; RIIT, Moscow), which may be used in the instrumentation for X-ray plasma diagnostics with a temporal resolution of 1 ns and better.