The oxidn. of n-butane (I) [106-97-8] at low and high hydrocarbon concn. is studied on a V(IV)-P (1:1) mixed oxide in relation to surface modification induced by the reaction medium. Butenes and butadiene [106-99-0] are formed from I at high I concn. with total consumption of inlet O; the formation of olefins from I is not obsd. when there is incomplete O consumption at low I concn. Under conditions in which formation of olefins is max., available evidence shows the presence in the catalyst of high amts. of V(III) together with V(IV) and the absence of V(V); in contrast, maleic anhydride (II) [108-31-6] is formed when lower amts. of V(III) together with both V(IV) and V(V) are found. It is suggested that 2 redox couples operate in the synthesis of II: (1) V(IV)-V(III) in the synthesis of olefins from I, and (2) V(V)-V(IV) in the synthesis of II from the olefins formed. At high I concns. and total O consumption, the lower d. of oxidizing sites for the second reaction limits the rate of further oxidn. of butenes to II, thus providing evidence for their formation from I. [on SciFinder(R)]