Understanding the reduction of MoO3 is important for the utilization of this material in optoelectronic applications. Here, we examine the changes that take place during reduction of a solution-processed thin film of MoO3. Upon reduction in dilute hydrogen gas, the nanocrystallite grain morphology is observed hosting a sequence of shear defects, nucleation and growth of reduced molybdenum oxide phases within the MoO3 matrix before decomposition of the film. The compositional changes are tracked and compared with changing optical properties. The formation of extended defects is associated with enhanced photoluminescence. The progression of reduction in the films is highly related to reduction of powders and should be considered when designing devices utilizing MoO3-x. (C) 2017 Elsevier B.V. All rights reserved.