The thermally ionized process of the Ti2+/Ti3+ donor level in Ti-doped ZnSe crystals grown by the vertical gradient freezing method was first analyzed with detailed isothermal capacitance transient spectroscopy. The ionization enthalpy and an apparent electron capture cross section of the trap observed from the Arrhenius plot of the emission time constant were 1.058±0.005 eV and (1.03±0.15)×10-12 cm2, respectively. The temperature dependence of the capture cross section of the ionized Ti3+ state revealed a lattice relaxation energy of 0.29±0.03 eV. The real depth of the donor level was determined to be 0.77 eV below the conduction band. © 1997 American Institute of Physics.