Musty off-flavour in drinking water is usually the result of contamination with geosmin (GSM), 2-methylisoborneol (MIB), 2-isopropyl-methoxypyrazine (IPMP), 2-isobutyl-methoxypyrazine (IBMP), 2,3,6-trichloroanisole (2,3,6-TCA), or 2,4,6-trichloroanisole (2,4,6-TCA), or in combinations of these substances. These compounds have low ppt (ng/L) thresholds in drinking water hence, their quantification is non-trivial. If present in the brewery's water, these substances can carry over into finished beer, imparting a musty taint to the product. In order to eliminate this defect, it would be advantageous to have a method for identification and quantitative determination of musties malodourants in both water and beer at the sub-threshold level. Numerous other components in the beer or water matrix, which interfere with analysis, can make quantification difficult. This poster will describe the method development and progress made towards identifying and quantifying all six analytes responsible for musty defects in beer, as well as water. The method uses L/L extraction combined with column clean-up to remove background interferences. Large sample volume injection is used in combination with GC-olfactometry for identification purposes. The same column clean-up and injection methodology, combined with stable isotope dilution GC-MS selected ion monitoring, is employed for quantification. At present, all six musty analytes can be identified in beer and water matrices at sub-threshold concentrations using this GC-olfactometric method. Quantification of GSM, 2,3,6-TCA, and 2,4,6-TCA has been realized for beer in the ppt (ng/L) range. This same method is applicable to water samples for quantification of GSM, 2,3,6-TCA, 2,4,6-TCA, and MIB in the ppt (ng/L) range.