Under Illinois' 1978 determinate sentencing law, each of seven felony classifications has a prescribed range of sentences, and the judge selects a sentence within the prescribed limits. Inmates must be released at the expiration of their sentences, less earned good time. To determine whether the law has had its intended effect, this study obtained sentencing data for the periods before and after the law's enactment. Since much of the law's impact was projected to occur at the correctional level, the implementation of determinate sentencing by correctional personnel was also reviewed. Procedures regulating good time, supervised release, and program participation reflect the continuation of discretionary decisionmaking and coerced treatment, which conflict with the objectives of early advocates of determinacy. Inmates do, however, have greater release certainty under the new law. Organizational and political factors account for the law's mixed success in meeting its original objectives. 5 tables, 13 notes, and a 57-item bibliography. (Author abstract modified)