Non-catalwc gas-sobd reactions are unportant m many matenal pmcessmg and chemtcsl mdustrtes Such reachons call for specud attentton today regardmg the coal gasdication processes hvlng the past fifteen years, a number of papers on gas-sohd reachons has appeared m the hterature Recently Szekely et al [l] pubbshed a book on the subject The bulk of the literature descnbe mathematical models that can be used to predict the umverston vs tune relattons durmg gas-solid reactions However, only few papers have been pubhshed contammg experunental data or expenmental techmques to test cnttcally the vabdtty of the ex#stmg models In the general case of a gas-sobd reaction, as the solid IS consumed, the pore structure opens up, mcreasmg the rate of dtffuston of reactant gas wttb tune Stmultaneously, the reactive area decreases and the reactlon rate decreases Consequently, to descnbe the relatton between area and solid volume, a model of the pore structure 1s reqmred Yag~ and Kunuf2,3] presented a sunple model of gas-sold reactions called shrmkmg-core model or shell progresswe model However, tlus model offers only a valid descnphon of reactions m which the sobd reactant IS unpervrous to the gaseous reactant or when the reamon 15 strongly dlffuston tontrolIed More general models assume the solid to be a pseudo-homogeneous matertal Some authors assume that the reactton rate IS nth order with respect to sobd concentration[4]. or propotional to the surface area of the sohd[5] In the latter case, tt IS necessary to spectiy a model of the pore structure Several models of pore structure have been pubhshed[fLlO] for lsothermai and nonisothermal gas-solid reacttons[l l-131 The models of pore structure used tn ttus work are (a) Petersen's [6] Random Pore Model, and (b) the umform packed spheres model or Gram model [7,8] The probkm of gas-solid reacttons can be conceptually reduced to a problem of stmukaneous iitffuston and reaction m a porous sohd with a tune dependent rate, which 1s mathemattcally analogous to the problem of dtffuslon m a solid catalyst under deacttvatton condtttons Recently, Petersen et al [14] and Petersen and Wolf[l5,16] have shown that the single pellet dtffuston reactor (SPDR) IS an expertmental techmque that allows one to dtscrmuna& among dtfferent potsonmg mechamsms The analogy, gas-s&d and catalyst deacttvatton. prompted the authors to utqture whether the SPDR could be used to test cntlcally the theorettcal models used m gas-solid reacbons The operatton and theory of the SPDR has been recently revtewed[17.18], consequently only a bnef descnptton unll be gwen here A schemata dtagram of a reacbve sobd pellet (catal* or n~talyttc) of slab geometry II shown m Fw l(a) Under d~Bus~on mfluencad