The paper describes two distinctive forma of thermal instability which can make their 8pp93u8nce in gas-solid reactions. The llrst is due to an unstable type of crossing of the curven which repreeent the rate of heat genemtion by reaction within the solid and the rate of heat loss from its surface. In thin connection the signiecance of the concept of ignition temperature is discueaed. The second type of instability ia one which affecta 1oaGeed regiona of the react@ solid ; under conditions where the temperature of the reaction interface becomes higher the larger the thickness of the layer of reaction product, any small initial variations in the thickne86 of this layer will rapidly augment themselves. A theory of this effect is put forward. R&urn&Lea auteura d&Gent deux formee difftkentes d'instabiliti thermique susceptiblea de me prod&e dana des &actions gaz-solides. La premi&re serait dQe B un type instable d'lntersection dee courbea : vit.eme de g&n&&ion de chaleur I?+ l'intkrieur du solide et vltesse de diperdition de chaleur p8r 88 surhx. Lets auteurs discutent, 13 ce sujet, du concept de la tempkature d'lgnition. La seconde forme d'lnstabilitk affecterait des r&ions bien locali&a du solide en &action ; si Is temp&ature de l'lnterface de r&&ion s'Q&ve, 1Upaisseur de la couche du produit de &action devient plus grande ; de petitea diifkences lnltiales dana l'@&aeur de c&k. couche s'ampliflent rapidement d'eties-m&mea. IAS autam approfondiasent cette &mike thkie. 1.