We investigated the photoionization mass spectrometry of clusters of alkali metal atoms (M = Li, Na and K) solvated with acrylonitrile (AN; CH2=CHCN) molecules to obtain the information on size-dependent stability of the clusters. In the photoionization mass spectra of M-m(AN)(n), strong ion signals were observed at M+(AN)(3). The M+(AN)(n) (n = 6 and 9) ions were also clearly observed as magic numbers in the mass spectra of M = Na and K. By comparison with the mass spectrum of cluster ions formed by ion-molecule reactions in the cluster source, these magic numbers were found to be due to the stability of neutral clusters. These results are explained by the intracluster electron transfer from an alkali metal atom to AN molecules followed by anionic oligomerization. The results of the calculation for Na(AN) based on density functional theory also supports this consideration.