Organic inhibitors, primarily in the form of dispersants, have been examined in laboratory screening and field pilot tests for silica scale control activity. Over 25 inhibitor formulations obtained from four different manufacturers were screened for scale control in a laboratory pressure reactor using a simple brine solution. Only a single inhibitor achieved > 50% reduction in scale deposition compared to control (no inhibitor) experiments. This inhibitor proved to be effective at a dosage of about 1 ppm; however, above that dosage, scale inhibition decreased and significant deposition was observed. It is postulated that overdosing the dispersants leads to flocculation. A few inhibitors yielded positive results, but scale reduction compared to control tests was < 35%. In contrast, progressive acidification of brine decreased scaling significantly. Several inhibitors were examined for scale control in a pilot test apparatus at a geothermal field. Two inhibitors decreased scale deposition by > 50%. The majority of inhibitors tested in the field exacerbated scaling. Similar to the laboratory tests, brine acidification inhibited scaling by 97%. From these results it appears that the use of organic silica scale inhibitors in geothermal operations is limited to only a few applications. Until the drawbacks of high inhibitor cost, generally poor effectiveness, exacerbation of scaling upon overdosing, and the long-term effect of dispersing silica colloid particles into injection formations are resolved, organic inhibitors will likely continue to see limited use in the industry. (C) 2002 CNR. Published by Elsevier Science Ltd, All rights reserved.