We report on a pilot study of a novel observing technique, defocused transmission spectroscopy, and its application to the study of exoplanet atmospheres using ground-based platforms. Similar to defocused photometry, defocused transmission spectroscopy has an added advantage over normal spectroscopy in that it reduces systematic errors due to flat-fielding, point spread function variations, slit-jaw imperfections and other effects associated with ground-based observations. For one of the planetary systems studied, WASP-12b, we report a tentative detection of additional Na absorption of 0.12 ± 0.03[+0.03] per cent during transit using a 2 Å wavelength mask. After consideration of a systematic that occurs mid-transit, it is likely that the true depth is actually closer to 0.15 per cent. This is a similar level of absorption reported in the atmosphere of HD 209458b (0.135 ± 0.017 per cent; Snellen et al. 2008). Finally, we outline methods that will improve the technique during future observations, based on our findings from this pilot study.