Advanced industrial robots are commanded to accomplish different tasks with program sequences that are executed in digital computers. The operating software within these computers provides users with information on positions and orientations of the end effectors by computing them as functions of the joint variables. These functions are generally not exact enough such that differences between the computed and the actual positions can be significant. Error sources that contribute to these differences for robots with rotary joints are examined. The effects of these errors are parameterized and measurement data are fitted to obtain the values of these parameters. It is concluded that with sufficient but not exhaustive detail in the error modeling the differences can be reduced significantly from 5.9-mm mean error with nominal model down to 0.28-mm mean error after error compensation.