Reaction products in clusters containing an alkali metal atom (M = Na, K, and Cs) and methyl propiolate (MP) molecules have been examined from results of photoionization efficiency (PIE) curve measurements. Two distinct humps were commonly observed in the PIE curves of Cs(MP)(n) for n greater than or equal to 3, whereas a relatively smooth curve was obtained for n = 2. In the photoionization mass spectra of M(MP)(n), an intensity anomaly at n = 3 was commonly observed as reported in the previous paper, which was due to a cyclotrimerization reaction producing a benzene derivative (trimethyl benzenetricarboxylate, BT). From the results of quantum chemical calculations for reacted and unreacted isomers, these two humps originate from two isomers with different ionization thresholds; the lower threshold is ascribed to a solvation-type isomer, CS(MP)(3), and the higher value comes from a reacted isomer composed of a cesium atom and a BT molecule. From the relative intensity of two isomers, the intracluster cyclotrimerization reaction producing BT is found to proceed efficiently in the present M-MP clusters.