MALDI mass spectrometry (MS) of 14- to 42-mer homogeneous oligonucleotides and their mixtures was carried out using a Vision 2000 instrument (Thermo BioAnalysis, Finnigan, United States). Conditions for the determination of oligonucleotide molecular masses were optimized by applying various matrices and operation modes. The most reproducible results with minimal uncontrolled decomposition of the oligonucleotides including their apurinization during the MALDI MS registration were obtained using 2,4,6-trihydroxyacetophenone as a matrix instead of 3-hydroxypicolinic acid, usually employed in the mass spectrometry of oligonucleotides. Our approach allows the determination of molecular masses of oligonucleotides obtained by chemical synthesis and the evaluation of their component composition and purity. It was applied to the mass spectrometric analysis of oliconucleotides containing a 3′-(methyl-C- phosphonate) group or a modified 1,N6-ethenodeoxyadenosine unit.