Cellulosic chars containing nickel species have been gasified in air in the temperature range 235-400 degreesC. The chars were prepared from Ni2+-exchanged carboxy methyl cellulose (NiCMC) by pyrolysis in N-2 at several heat treatment temperatures (HTTs) ranging from 400 to 900 degreesC. All the chars displayed a dramatic jump in reactivity when the gasification temperature (GT) was raised to a particular value (jump temperature, T-j). Above T-j, the rate did not vary significantly with a further increase in temperature. T-j increased with HTT. The jump was accompanied by a transition from a rate-controlled, low reactivity, high activation energy region to a diffusion-controlled, high reactivity, low activation energy region. Chars of pure CMC did not show such a jump in gasification rate with increase in GT. XRD studies showed that elemental nickel is present in chars of HTT greater than or equal to 500 degreesC, in agreement with thermodynamic calculations. A sudden enormous increase in the mobility of the catalyst species in the neighborhood of it;; Tammann temperature (T-Tam) appears to be the cause for the jump in char reactivity. This jump strongly influenced the dependence of char gasification rate on HTT, affecting the usual monotonic decrease in rate with an increase in HTT. A monotonic decrease was observed only when the GT lied below the T-j of the char. When GT greater than or equal to T-j, the rate did not decrease with an increase in HTT, but rather increased or remained almost invariant. On the other hand, a monotonic decrease in rate with an increase in HTT was obtained in the absence of nickel species in the char. The study emphasizes the important role of mobility of catalyst species in controlling the char reactivity.