A laboratory scale desupersaturation unit (DU) was used to precipitate barium sulfate from supersaturated reverse osmosis (RO) concentrates. The effect of pH and Natural Organic Matter (NOM) on crystal growth were investigated. Effective crystallization was observed in the absence of NOM (Synthetic concentrate). Two RO concentrates from surface water treatment plants (organic matter content of 12 and 8 mg/L TOC, respectively) were examined for ca. 200 hours. One precipitated readily in the DU and was not affected by pH (3.5 to 8.0), while the other showed break-through at increasing pH (ca.30 h at pH=6.7). Retarded crystal growth was attributed to the different nature of NOM rather than the TOC concentration. Characterization of organic matter showed the dominant fraction of humics (ca.60%). "Building blocks", free organic acids, amphiphilic compounds and small amount of polysaccharides also were present. On-going research is directed at isolation and characterization of readily adsorbing fraction of NOM on crystallization surfaces. Includes 10 references, tables, figures.