A process for controlling an oxygenates to olefin reactor is disclosed which comprises: contacting oxygenate-containing feedstock, e.g., methanol, in a reaction zone in the presence of a molecular sieve oxygenate to olefins conversion catalyst under conditions sufficient to provide an olefins-containing effluent containing alkyl alcohol and dialkyl ether, e.g., methanol and dimethyl ether; analyzing a single gas phase of the effluent determine alkyl alcohol concentration and dialkyl ether concentration; and adjusting reactor conditions, e.g., WHSV, as a function of alkyl alcohol concentration and dialkyl ether concentration, as necessary to provide a substantially consistent effluent composition. A corresponding apparatus for carrying out the process is also provided.