TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Catalyst: The catalyst catalyzes an alkylation reaction, a transesterification reaction, an esterification reaction, an oligomerization reaction, a polymerization reactor, or an isomerization reaction. The catalyst includes an acid functionality or a base functionality. It is partially deactivated by hydrogen deficient fouling agents.Preferred Method: Contacting the catalyst with a fluid reactivating agent that is at or above its critical point and is of density to dissolve impurities comprises contacting the catalyst with the fluid reactivating agent that is capable of transferring a hydride ion to the fouling agent, or contacting the catalyst with the fluid reactivating agent comprising an alkane having at least one tertiary carbon atom or a compound that can be isomerized in the presence of the catalyst to form at least one tertiary carbon atom. It involves contacting the catalyst with the fluid reactivating agent at a pressure equal to a critical pressure and at a temperature equal to a critical temperature. It involves contacting the catalyst with isobutane at 100-300degreesC and 200-5000 psig. Reacting the fluid reactivating agent with the fouling agent involves transferring a hydride ion from the fluid reactivating agent to the fouling agent. Releasing the fouling agent from the catalyst comprises releasing the fouling agent having a molecular weight approximately equal to or higher than that of the fouling agent deposited on the solid catalyst. Releasing the fouling agent from the catalyst involves desorbing the fouling agent from the solid catalyst and dissolving the fouling agent in the fluid reactivating agent. Precipitating the fouling agent comprises altering solubility properties of the fluid reactivating agent. Recycling the fluid reactivating agent comprises reusing the fluid reactivating agent as at least a portion of a feed mix and a second portion of the fluid reactivating agent as a reactivating agent.Preferred Compounds: The fluid reactivating agent comprises isobutane, isopentane or their mixtures. It comprises n-butane, n-pentane or their mixtures. It may comprise benzene, toluene, ethylbenzene or their mixtures. It comprises a dissolved species of hydrogen or oxygen.