Abstract: A vacuum evaporator/sublimator flask apparatus comprises an elastomer cap and a cylindrical glass body. The glass body is a straight-sided cylinder, preferably made of borosilicate glass tubing of sufficient wall thickness to prevent implosion when subject to high vacuum, open at one end and sealed at the other. The elastomer cap is preferably fabricated of clear elastomer in a one-piece molded arrangement which includes an upper neck portion, an outwardly extending sloping intermediate portion terminating in outer downwardly extending walls having an inwardly extending flange at the end thereof to engage the glass body. The neck portion has a tubular opening and a molded-in-place insert at the base of the neck to serve as a stop for rigid tubing that is inserted to permit connection to a vacuum system. The upper portion of the neck includes an inwardly extending portion which engages a vacuum insert.