An Optical Backscatter Reflectometer (OBR) with optical frequency domain reflectometry (OFDR) enables measurement of the spatial temporal temperature inside catalytic reactors with unparalleled combined spatial and temporal resolution. It was used to measure the transient temperature inside a single channel of a washcoated monolith reactor in which the Pt-catalyzed oxidation of ammonia was carried out. Measurements of the monolith temperature response to either a rapid step decrease in the feed temperature or an increase of the flow rate were used to test the capability of the OFDR technique and to determine their impact on the transient temperature rise of the monolithic catalyst. The rapid step increase in the flow rate can generate a transient peak temperature of the solid phase higher than that at the steady state. This transient temperature rise was smaller than that caused by a sudden decrease of the feed temperature. (C) 2013 Elsevier B.V. All rights reserved.