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Radon, thoron and their progeny are present in the indoor atmosphere as attached and unattached fractions. The level of ventilation in a house and the plating out of radon/thoron daughters onto surfaces decide the extent of equilibrium between them. Since the daughter products are mainly responsible for the inhaled dose, the measurement of the equilibrium factor, F, between radon/thoron and their progenies is desirable. It is therefore necessary to measure the equilibrium factor for different types of house construction, locations and seasons before projecting the dose from the measurements of radon and thoron. This paper presents the results of the measurement of F between radon, thoron and their progenies in dwellings using the LR-115 type II plastic track detector. The exposures were made with a twin-chamber dosimeter in about 150 houses of different types, distributed at different locations in the Garhwal and Kumaun region of the Himalayas in India. The measured equilibrium factor between radon and progeny varied from 0.02 to 0.90 with an average value of 0.28 while the same factor for thoron and progeny was found to vary from 0.01 to 0.90 with an average value of 0.09. The methods of measurement and results obtained are discussed in detail.