Thoron gas and its progeny behave quite differently in room environments, owing to the difference in their half-lives; therefore, it is important to measure simultaneously gas and progeny concentrations to estimate the time-integrated equilibrium factor. Furthermore, thoron concentration strongly depends on the distance from the source, i.e. generally walls in indoor environments. In the present work, therefore, the measurements of both thoron and radon gas and their progeny concentrations were consistently carried out close to the walls, in 43 dwellings located in the Sokobanja municipality, Serbia. Three different types of instruments have been used in the present survey to measure the time-integrated thoron and radon gas and their progeny concentrations simultaneously. The equilibrium factor for thoron measured ‘close to the wall’, F^W_Tn, ranged from 0.001 to 0.077 with a geometric mean (GM) [geometric standard deviation (GSD)] of 0.006 (2.2), whereas the equilibrium factor for radon, F_Rn, ranged from 0.06 to 0.95 with a GM (GSD) of 0.23 (2.0).