Trihalomethanes (THMs) are known to be generated from specific fractions of natural organic matter (NOM) present in surface waters used for potable water supply. The overall specific THM formation propensity (THMFP) can be expressed in terms of yield - the mass of THM generated per unit mass of permeate total organic carbon (TOC). A study of permeate water quality, with specific reference to TOC rejection and THM yield, has been conducted for membrane-based potable water supply plants across Scotland. The study encompassed 35 operating plants fitted with "tight" UF membranes, and was based on 18 months of water quality (WQ) data. Literature data used to calculate yield for comparable pilot or full scale membrane plants challenged with various real waters and operating under similar conditions indicate permeate THM yields predominantly in the range of 20-50 mu g total THMs (tTHMs) per mg TOC across a range of membrane characteristics. Outcomes from the study revealed average yields within the range of 10-70 mu g THMs/mg TOC, with TOC passage averaging 18%, roughly in keeping with published values for comparably selective membranes. Data for THM distribution revealed that, counter-intuitively, increased bromine substitution lead to slightly reduced total THM levels. (C) 2015 Elsevier B.V. All rights reserved.