This paper analyzes assumptions that are implicitly made in updating estimates of nominal values for environmental goods produced by previous valuation studies, especially adjustments made through use of the consumer price index (CPI). The paper illustrates that the underlying form of preferences for the selected values from a past study may have a significant influence on the updated environmental values. We show why care is necessary in updating past welfare or value estimates in the face of changes in the demand and supply for environmental amenities when using the CPI. ¿ 1997 American Geophysical Union