The modular gas cylinder storage system of the present invention includes several gas cylinder holding devices which are incorporated into an overall cylinder holding system which holds different-sized cylinders. It includes a rack structure that is modular and designed to hold large tanks. A plurality of smaller cylinders are held in several differing basket configurations which basket configurations are formed to mate with the modular rack structure. Barriers for dividing cylinders holding incompatible chemicals are held in place by brackets attaching the barriers to the rack structure. Secure fastening of large cylinders is achieved utilizing a gas cylinder gas storage bracket which is bolted to other structural members having a concave face which mates with the curved surface of the gas cylinder. A flexible strap with an adjustabe buckle passes through the bracket and around the cylinder for a tight, secure engagement of the cylinder within the bracket.