AbstractCalendered sheets of pale crepe rubber containing an ultra‐accelerator, piperidiniumN‐pentamethylenedithiocarbamate, and zinc oxide were embedded in powdered sulfur and kept at room temperature. Tensile strengths up to 294.2 kg./cm.2(4185 psi) and elongations up to 530% were attained; the combined sulfur in 115 weeks was 3.60%. Calendered pale crepe rubber containing 10 parts of yellow mercuric oxide to 100 parts of rubber allowed to stand covered with quinone dioxime (GMF) powder at room temperature slowly vulcanizes, reaching a tensile strength of 53.1 kg./cm.2(754 psi) in 405 days, and retaining this condition for at least twice this same period of time. The elongation reached 740%, and the set was 0.16. The tensile strength obtained is almost as high as that obtained in a press cure with quinone dioxime alone. The untreated sample containing yellow mercuric oxide alone showed a tensile strength of only 5.3 kg./cm.2(75 psi). Calendered pale crepe with no mercuric oxide set in quinone dioxime and calendered pale crepe containing 2 parts of quinone dioxime, when allowed to stand at room temperature, showed no signs of vulcanizat