Decay dynamics of photoexcited Eu ions in CaS:Eu and in CaS:Eu,La were studied by employing time-resolved luminescence spectroscopy. For the first time, subnanosecond decay process of Eu3+ luminescence was observed in solid matrices. The obtained decay times of Eu3+ in CaS:Eu,La were 4.3 ns at 298 K and 0.38 ns at 77 K, demonstrating much faster non-radiative decay at the lower temperature. The overall intensities of the emission spectra at 298 K were three times stronger than that at 77 K. The formation of Eu3+-La3+ pairs in CaS:Eu,La accounts for observed dependence of fast decay time and intensity on the temperature. The decay time of Eu2+ both in CaS:Eu and CaS:Eu,La was observed to be approximately 560 ns.