Systematic studies of the fluorescence picosecond lifetimes determination by laser-excited time-correlated single-photon-counting (TCSPC) have been undertaken. The results have been used to develop methods for determining lifetimes with much smaller error and much greater reproducibility than any hitherto reported. The error in the determination of the lifetimes ( three standard deviations) can be as low as that in simulations and amounts to 0.01 FWHM of the IRF. The lifetimes determined for the second excited singlet state of xanthione in toluene (5.1 +/-0.3 ps) and in benzene (8.1 +/-0.3 ps) can be treated as reliable standards of picosecond lifetimes.