Positron annihilation lifetime spectroscopy as a technique of porousmaterials investigation is still under development. Both measurement con-ditions and lifetime spectra analysis methods have to be adapted to therequirements of positron porosimetry. If complex positron annihilation life-time spectra of porous materials are processed, numerical methods of life-time spectra analysis have to be used with a particular care. Among variousmethods of positron annihilation lifetime spectra analysis available, the mostpopular ones were tested implemented in programs MELT and LT. It wasfound, from the simulated spectra study, that a large number of counts ina spectrum is needed to obtain reliable results of analysis. Even then onlyan approximate solution is available. Determination of the approximationnature helps in a further interpretation of the results and in positron an-nihilation lifetime spectrometer setting adequate to the problem. For thispurpose, a few complex spectra obtained from positron beam facility weretested. The results show a negligible in°uence of resolution function on longlifetime components, while the precise estimation of the background level iscrucial. Also some distortions caused by imperfection of numerical analysismethods are pointed out.PACS numbers: 78.70.Bj, 36.10.Dr, 61.43.Gt, 81.07.{b, 41.75.Fr