The first-order rate of softening of cucumber tissue containing 1.5M NaCl with and without 20 mM added calcium ion was determined from pH 2.5 to 8.6 at 74°C. Calcium ion had little inhibitory effect on rate of softening when pH was above 5. Below pH 5 the relative effectiveness of calcium in reducing rate of softening increased as pH decreased. This behavior was opposite to that expected if calcium inhibited softening by cross-linking negatively charged pectin car-boxyl groups. This was more evidence against that mechanism as an explanation for inhibition of softening by calcium ion in plant tissues. A very large decrease in rate of tissue softening was observed at alkaline pH whether or not calcium was added.