Contaminated sites by NAPL (Non-Aqueous Phase-Liquids) may lead to safety risks to human health and to ecosystems, restrictions to urban development and decrease of real estate value. This work used the radon gas as an indicator for the analysis of subsurface soil gas, once this noble gas presents good solubility in a wide range of NAPL, being partially retained in the NAPL contamination. Therefore, a decrease of the activity of radon in the contaminated soil gas can be expected, due to the high capacity of partitioning of radon in NAPL, which allows that the NAPL retain part of the radon previously available in the soil pores. The survey was carried out at a disused industry, contaminated by low volatile NAPL, located at southeast of São Paulo city, from June/14 to May/15. Radon was evaluated by passive detection methodology with CR-39 solid state nuclear track detectors (SSNTD) in ten monitoring stations installed in the contaminated area investigated and named "A" to "J". Radon concentrations average for the eight monitoring stations at non-contaminated locations varied from (22 ± 4) kBq.m-3 to (39 ± 4) kBq.m-3. For the two monitoring stations assumed as contaminated locations, radon concentrations average were (1.4 ± 0.4) kBq.m-3 and (13 ± 9) kBq.m-3. The results have shown good agreement between the used method and the conventional environmental investigation techniques, for the majority of the monitoring stations in different seasons. Results obtained with CR-39 detectors varied over the exposure time due to the different seasons. No relation was observed between radon activity concentrations and rain volume accumulated over the different CR-39 exposure times. The lowest 222 Rn activity concentrations occurred in "G" and "H" monitoring stations, also verifying by gamma-ray spectrometry, that the low activities are not related to the activity concentration of its father 226 Ra from the 238 U decay chain, reinforcing the theory that radon gas is retained in sites where NAPL contamination is present. Results obtained during environmental remediation proved that the methodology employed in this study was more efficient than the conventional investigation techniques, specially for the "D" and "G" monitoring stations to the investigated site.