Ubiquitination is a widely-studied regulatory modification involved in protein degradation, DNA damage repair and the immune response. Conjugation of ubiquitin to a substrate lysine occurs in an enzymatic cascade involving an E1 ubiquitin activating enzyme, and E2 ubiquitin conjugating enzyme, and an E3 ubiquitin ligase. Assays for ubiquitin conjugation include electrophoretic mobility shift assays and detection of epitope-tagged or radiolabeled ubiquitin, which are difficult to quantitate accurately and are not amenable to high throughput screening. We have developed a colorimetric assay that quantifies ubiquitin conjugation by monitoring pyrophosphate released in the first enzymatic step in ubiquitin transfer, the ATP-dependent charging of the E1 enzyme. The assay is rapid, does not rely on radioactive labeling, and requires only a spectrophotometer for detection of pyrophosphate formation. We show that pyrophosphate production by E1 is dependent on ubiquitin transfer and describe how to optimize assay conditions to measure E1, E2, or E3 activity. The kinetics of polyubiquitin chain formation by Ubc13-Mms2 measured by this assay are similar to those determined by gel assays, indicating that the data produced by this method are comparable to methods that measure ubiquitin transfer directly. This assay is adaptable to high-throughput screening of ubiquitin and ubiquitin-like conjugating enzymes.