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A comparative study of five 5 × 5 × 5 mm3 LSO:Ce samples with different co-dopings of calcium, from 0 to 0.4% is reported. All the results are compared to those obtained with selected 10 × 10 × 5 mm3 LSO crystal, tested in previous studies. The influence of Ca co-dopant on scintillator parameters was studied using a Photonis XP20D0 fast timing photomultiplier. The tests included measurements of light output in terms of number of photoelectrons, measurements of time resolution in coincidence experiments with annihilation quanta from a 22Na gamma source, and decay time constant calculations on the basis of timing spectra obtained using the Bollinger-Thomas single photon method. Also energy resolution for 662 keV from 137Cs gamma source is mentioned at the end of the paper. The results showed the significant influence of the Ca co-dopant on the properties of the LSO scintillators. For low Ca concentrations of 0.1%, photoelectron number and energy resolution were improved up to 7800 phe/MeV and 7.3 % respectively. On the other hand the shortest decay time constants were obtained for higher concentrations of 0.3% and 0.4% of Ca giving values equal to about 30 ns. The time resolution improvement was similar for all the samples and calculated values for a single detector were equal to about 140 ps.