A process for producing unsaturated carboxylic acids or unsaturated nitriles by vapor phase oxidation reaction of their corresponding C 3 to C 5 alkanes, C 3 to C 5 alkenes, and mixtures thereof, as a hydrocarbon starting material, using a reaction system having at least two reaction zones arranged in series with one another and at least one catalyst capable of catalyzing the vapor phase oxidation reaction disposed in each of the at least two reaction zones, wherein at least one intermediate effluent stream exits a preceding one of the at least two reaction zones and is at least partially fed to a subsequent one of the at least two reaction zones, said process comprising the steps of: feeding an initial feed stream comprising the hydrocarbon starting material into a feed point of a first reaction zone of the at least two reaction zones, separating the at least one intermediate effluent stream from the first reaction zone into at least an intermediate product stream comprising an oxidation product selected from the group consisting of an unsaturated carboxylic acid and an unsaturated nitrile, and an intermediate feed stream comprising at least one hydrocarbon starting material selected from the group consisting of an unreacted C 3 to C 5 alkane, an unreacted C 3 to C 5 alkene, and mixtures thereof; feeding an oxygen-containing gas and the intermediate feed stream to a subsequent reaction zone; the process further comprising: a. determining the activity of the catalyst in at least one reaction zone; b. monitoring the activity of the catalyst in each zone to determine the time when the catalyst in any zone has lost at least 25% of its activity; c. after step b., altering the flow path of at least one of the feed streams such that at least one of the hydrocarbon starting material or the intermediate feed stream is fed to a new feed point, the new feed point being either a feed point of a different reaction zone or a different feed point of the same reaction zone, wherein the catalyst at that new feed point has a higher activity than the activity of the catalyst at the feed point to which the relevant feed was fed prior to this step c. feeding an initial feed stream comprising the hydrocarbon starting material into a feed point of a first reaction zone of the at least two reaction zones, separating the at least one intermediate effluent stream from the first reaction zone into at least an intermediate product stream comprising an oxidation product selected from the group consisting of an unsaturated carboxylic acid and an unsaturated nitrile, and an intermediate feed stream comprising at least one hydrocarbon starting material selected from the group consisting of an unreacted C 3 to C 5 alkane, an unreacted C 3 to C 5 alkene, and mixtures thereof; feeding an oxygen-containing gas and the intermediate feed stream to a subsequent reaction zone; the process further comprising: a. determining the activity of the catalyst in at least one reaction zone; b. monitoring the activity of the catalyst in each zone to determine the time when the catalyst in any zone has lost at least 25% of its activity; c. after step b., altering the flow path of at least one of the feed streams such that at least one of the hydrocarbon starting material or the intermediate feed stream is fed to a new feed point, the new feed point being either a feed point of a different reaction zone or a different feed point of the same reaction zone, wherein the catalyst at that new feed point has a higher activity than the activity of the catalyst at the feed point to which the relevant feed was fed prior to this step c. the process further comprising: a. determining the activity of the catalyst in at least one reaction zone; b. monitoring the activity of the catalyst in each zone to determine the time when the catalyst in any zone has lost at least 25% of its activity; c. after step b., altering the flow path of at least one of the feed streams such that at least one of the hydrocarbon starting material or the intermediate feed stream is fed to a new feed point, the new feed point being either a feed point of a different reaction zone or a different feed point of the same reaction zone, wherein the catalyst at that new feed point has a higher activity than the activity of the catalyst at the feed point to which the relevant feed was fed prior to this step c. a. determining the activity of the catalyst in at least one reaction zone; b. monitoring the activity of the catalyst in each zone to determine the time when the catalyst in any zone has lost at least 25% of its activity; c. after step b., altering the flow path of at least one of the feed streams such that at least one of the hydrocarbon starting material or the intermediate feed stream is fed to a new feed point, the new feed point being either a feed point of a different reaction zone or a different feed point of the same reaction zone, wherein the catalyst at that new feed point has a higher activity than the activity of the catalyst at the feed point to which the relevant feed was fed prior to this step c.