Humic substances (HS) play an important role in the phototransformation of micro-pollutants in surface waters and a significant part of this reactivity is often attributed to oxidant triplet excited states (3HS*). The present study aims to understand why 3HS* were found to be negligibly involved in the HS-mediated phototransformation of β2-adrenoceptor agonist terbutaline (TBL) while this chemical shows a high reactivity towards triplet 3-carboxybenzophenone (3CBP*), a typical HS model sensitizer. To clarify this apparent discrepancy, several experiments were conducted. We first confirmed that TBL can be easily oxidized by triplet riboflavin (3RF*), another sensitizer (k = (1.7 ± 0.4) × 109 M−1 s−1). Afterwards, we studied the effect of TBL on the HS-mediated phototransformation of 2,4,6-trimethylphenol (TMP), chosen for its high reactivity with 3HS*. TBL was found to enhance the rate of TMP loss. This result is rationalized by postulating that (i) TMP and TBL are both oxidized by 3HS* to yield the phenoxyl radicals TMP-H• and TBL-H•, respectively, and (ii) TBL-H• is further reduced by TMP which generates additional TMP-H•. Fitting the experimental data gives k = (1.8 ± 0.5) × 109 M−1 s-1 for the reaction between 3HS* and TBL and DFT calculations further support the conclusion that TBL-H• can be reduced by a lot of phenols. Hence, in the case of TBL, the oxidant effect of 3HS* is counterbalanced by the antioxidant properties of HS.