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Total dissolved solids (TDS, in mg/ℓ) can be estimated from measurements of electrical conductivity at 25 C (EC, in μS/cm) by applying a conversion factor f. This factor is commonly reported to range from 0.54 to 1.1. For 45 South African mine-water samples, factors between 0.25 and 1.34, with a median of 0.85, were determined. The samples cover an EC-range of 70–16 000 μS/cm and TDS of 50–14 000 mg/ℓ. Linear regression for the entire dataset yields a conversion factor of 0.88 but for samples with EC< 5 000 μS/cm, a conversion factor of 0.97 is recommended. However, both of these factors allow only estimates of TDS and for accurate TDS values it is necessary to determine the conversion factor specifically for each site. Besides spatial variations, temporal variations of conversion factors were also observed.