An apparatus for multiple simultaneous synthesis (110), having a number of vials (28), each vial (28) having an upper end (31), a lower end (30), and a barrel (29), the upper end (31) opening into a mouth (32). It also includes a lower frame assembly (181) having a number of holes (101, 103, 105) for receiving the vials (28), and an upper frame assembly (180), which includes inner side-walls (195) and a ceiling portion (190). A gas-tight seal (197) surrounds a plenum (85), the plenum (85) being formed between the ceiling portion (190) of the upper frame assembly (180) and the lower frame assembly (181). The plenum (85) is adjustable in volume by the positioning of the upper frame assembly (180) in variable relation to the lower frame assembly (181), while maintaining the gas-tight seal (197). The volume of the plenum (85) is adjustable to a minimum whereby the mouths (32) of the vials (28) are sealed by the ceiling (190) of the gas-tight seal (197), which is optionally an upper gasket (90). Also disclosed is a plenum enclosure (182) for multiple simultaneous synthesis used in conjunction with vials (28), which includes the upper frame assembly (180) and lower frame assembly (181) and the gas-tight seal (197), as well as a gasket vial holder (185) to be used in the plenum enclosure (182).