In situ X-ray absorption spectroscopy (XAS) under reaction conditions of selective propene oxidation was employed to elucidate the local structure of as-prepared and activated molybdenum oxide supported on hollow vapor-grown carbon nanofibers (VGCNF). The local structure of as-prepared MoxOy-VGCNF was very similar to that of hexagonal MoO3. During heat treatment in propene- and oxygen-containing atmosphere, as-prepared MoxOy-VGCNF transforms into activated MoxOy-VGCNF above 623 K. The local structure around the Mo centers in activated MoxOy-VGCNF is similar to that of alpha-MoO3. Temperature- and time-dependent XAS measurements showed a rapid transformation from hex-MoO3 to alpha-MoO3 supported on VGCNF under reaction conditions. Subsequently, the resulting activated MoxOy-VGCNF catalyst exhibited a slowly increasing average oxidation state. The latter coincided with the formation of acrylic acid, which is hardly detectable during catalysis on regular, binary alpha-MoO3. Moreover, activated MoxOy-VGCNF is much more active in the selective oxidation of propene compared to alpha-MoO3. The correlation between catalytic selectivity and average oxidation state as a result of suitable reduction-oxidation kinetics corroborates the importance of structural complexity rather than chemical complexity. (C) 2010 Elsevier Inc. All rights reserved.