Size-dependent stability and intracluster reactions have been investigated by photoionization mass spectrometry for alkali metal (M = Li, Na, and K)-methacrylate (CH2=C(CH3)CO2R; R=CH3 and C2H5) clusters. The following two features are commonly observed in the mass spectra of M(CH2=C(CH3)CO2R)(n) clusters; (1) enhanced ion signal (magic number) at n = 3, and (2) fragment ions with a loss of ROH only for n = 3 and 2. In addition, the magic number behavior at n = 6 is also observed in K system. These features are explained by an intracluster oligomerization and following dissociation reactions. The magic number trimer has a cyclohexane structure as a result of oligomerization, and the two dissociative products also have cyclic structures. The loss of ROH from n = 3 and n = 2 cluster can be attributed to Dieckmann cyclization and lactonization reaction, respectively. From the size-dependent additivity of H2O to M(methacrylate)(n) clusters, the numbers of methacrylate molecules filling the first solvation shell for Li, Na, and K systems are estimated to be 4, 5, and 6, respectively. From the consideration of the intracluster oligomerization, it is suggested that one and two methacrylate molecules directly bind to the metal atom in Li and Na systems, respectively, in addition to one cyclohexane derivatives. In the K system, two cyclohexanes are suggested to bind directly to the K atom, which is consistent with the observation of the n = 6 magic number.