The purpose of this bulletin is to raise the awareness of the hazards associated with cleaning of diborane and pentaborane cylinders, and of disposing of or hydrotesting diborane and pentaborane cylinders that have not been properly cleaned. Extreme caution should be observed. Do not dispose of or hydrotest a diborane or pentaborane cylinder unless it has been properly cleaned and decontaminated. It is important to realize that empty cylinders contain hazardous product residue. Diborane, pentaborane, and their decomposition product residue can react with air and water, creating products that could overpressure a cylinder. Serious injury to personnel and property can result. Cylinders can burst emitting highly toxic substances that can cause fatalities. Only specially trained and qualified personnel should attempt to clean or decontaminate a diborane or pentaborane cylinder. Pentaborane has not been commercially available since the 1980s.