Dobson spectrophotometer measurements are known to be subject to stray light errors. To study these errors with Dobson spectrophotometer no. 188 in Athens, Greece, a series of total ozone measurements were taken out to high airmasses extending as far as mu = 3.0. The results were compared to the simple monochromatic-heterochromatic stray light model introduced by Basher. These comparisons showed that the instrument's Ro value was between 10-3.5 and 10-3.4. These Ro values indicate that the instrument does not introduce any stray light interference for the regular observations performed at Athens. The value of the other parameter, a, of the Basher model varied between 0.6 and 0.9, having a weak dependence on the total ozone amount.