Twelve practicing petroleum engineers participated in a study comparing deterministic with probabilistic reserve estimates. Production data for five wells were provided sequentially in four phases covering the first 5 years of production history. A deterministic and probabilistic reserve estimate was made by each evaluator for each phase. The deterministic reserve estimates were based on Securities & Exchange Commission (SEC) definitions. For the probabilistic reserve estimates, the evaluators were asked to supply three values: P-90, P-50, and P-10. P-90 is the value where there is a 90% probability that the actual reserves are greater than this value; P-50 is the value where there is a 50% probability that the actual reserves are greater than this value; and P-10 is the value where there is a 10% probability that the actual reserves are greater than this value. Most evaluators used their deterministic estimate as the basis for the probabilistic estimates. in these cases, subjective estimates were made rather than using a statistical approach. The evaluators have more difficulty in estimating the P-90 value when the decline exponent b is low. The evaluators have the same difficulty in estimating the P-10 value when the decline exponent b is high. In comparing the individual evaluators, there is considerable variability in deterministic, P-90, P-50, and P-10 values. The most variability is in the P-10 values. Also, it was observed that over time, as more production history is recorded, P-90 values do not approach truth from the low side.