Many organic granular materials are susceptible to self-heating when stored in silos. Under the right environmental conditions self-heating can lead to spontaneous ignition and smoldering combustion. This is a case study of an explosion that occurred in a grain storage silo with a smoldering fire. Although the hazards of combustible dust are relatively well recognized, the explosion hazard presented by a smoldering fire is less well known. There were two explosions in this incident: a primary explosion involving carbon monoxide and smoke generated by the smoldering fire, and a secondary explosion fueled by combustible dust. The explosion caused both injuries to personnel and significant property damage. This article discusses the causal factors for the explosion and the lessons learned for effectively monitoring and responding to a smoldering fire in a silo. The potential explosion hazard of the carbon monoxide and smoke generated by a smoldering fire is emphasized. (c) 2013 American Institute of Chemical Engineers Process Saf Prog 33: 94-103, 2014