To study the freezing/melting behavior of a confined N-2, CO, and Kr, we performed X-ray diffraction measurements of NZ, CO, and Kr confined inside the cylindrical pores of five kinds of siliceous MCM-41 with different pore radii (R = 1.45-2.9 nm) as a function of temperature. Freezing and melting of these three molecules were almost reversible for all the MCM-41 samples used here. Freezing of the CO confined to the mesopores resulted in the appearance of the orientationally ordered alpha phase, whereas the N-2 solidified into the orientationally disordered phase analogous to the bulk beta phase. The solid Kr confined to the mesopores contains a considerable amount of random stacking faults. The freezing-point depressions of N-2 and CO relative to the bulk melting point were nearly the same and were slightly larger th;ln that of Kr. Upon cooling, both molecular liquids of Nz and CO confined to the cylindrical mesopores of R = 2.1 nm vitrify, whereas liquid Kr crystallizes within the mesopores of R = 1.8 nm and vitrifies within the mesopores of R = 1.45 nm, The mechanism of vitrification of a confined liquid upon cooling is considered.