The present study illustrates the influence of relative humidity (RH) and applied voltage on the tribo-electrostatic separation of ash from Indian coking coal washery rejects. A prototype system was designed, fabricated, and installed to carry out the separation studies. The rejects on an average contained 51% ash; 33% fixed carbon and 16% volatile matter. The contact electrification of carbon and ash forming minerals was carried out using a rotary drum tribo-charger. The results indicate that the absolute charge magnitude of these particles is different and dependent on the operating variables. The charge magnitude of the particles decreases with an increase in humidity and a higher differential charge between carbon and mineral matter can be struck at lower RH. The results also show that the charge magnitude increases with an increase in the tribo-charging time. The influence of environmental moisture on the tribo-electrostatic separation of coking coal was analysed. The results show that the yield and quality of the clean coal decreases with an increase in humidity but increases with an increase in applied voltage. The optimum separation efficiency is achieved at 10% RH, and 15 kV applied DC voltage. The overall results reflect that it is possible to produce clean coal with 35% ash at 35% yield from the feed sample (rejects) with 53% ash. (C) 2016 Elsevier B.V. All rights reserved.