Pure crude rubber is a white or colorless hydrocarbon. The simplest unit of rubber is isoprene, which has the chemical formula C5H8. At the temperature of liquid air, which is about -195° C (about -319° F), crude rubber is a hard, transparent solid; from 0° to 10° C (32° to 50° F) it is brittle and opaque, and above 20° C (68° F) it becomes soft, resilient, and translucent. When rubber is mechanically kneaded, or is heated above 50° C (122° F), it becomes plastic and sticky; above 200° C (392° F) it decomposes.

Crude rubber is insoluble in water, alkali, and weak acid; it is soluble in benzene, gasoline, chlorinated hydrocarbons, and carbon disulfide. It is oxidized readily by chemical oxidizing agents, and slowly by atmospheric oxygen.