In recent years, polar solvents such as dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) have received a considerable attention as a promoter of organic reactions. We have developed several catalyst-free reactions using DMSO and molecular sieves (MS) 4A. Very fundamental and significant reactions in organic synthesis, such as Henry reaction, Michael addition reaction, Knoevenagel reaction and so on, proceed smoothly at room temperature in DMSO with MS 4A to afford the corresponding adduct in high yields. By this method, neither a base nor an acid catalyst is necessary to complete the reaction. These reactions shown here are attractive from the standpoints of environmentally friendly organic synthesis because of high atom economy and mild reaction conditions.