The Adirondack Mountain region of New York, a historical hotspot for atmospheric sulfur and nitrogen deposition, features abundant lakes that are experiencing browning associated with recovery from acidification. Yet, much remains unknown about the photoreactivity of Adirondack lake waters. We quantified the apparent quantum yields (Φapp,RI) of photochemically produced reactive intermediates (RIs), such as excited triplet states of dissolved organic matter (3DOM*), singlet oxygen (1O2), and hydroxyl radicals (•OH), for surface waters collected from 16 representative Adirondack lakes. Φapp,3DOM* and Φapp,1O2 for native Adirondack lake waters fell within ranges reported for whole waters and DOM isolates from various sources, while Φapp,•OH were substantially lower than those measured for other aquatic samples. Orthogonal partial least squares and multiple linear regression analyses identified the spectral slope coefficient from 290 to 400 nm (S290-400) as the most effective predictor of Φapp,RI among measured water chemistry parameters and bulk DOM properties. Φapp,RI also exhibited divergent responses to controlled pH adjustment and aluminum or iron addition simulating hypothetical scenarios relevant to past and future water chemistry conditions of Adirondack lakes. This study highlights the need for continued research on changes in photoreactivity of acid-impacted aquatic ecosystems in response to browning and subsequent impacts on photochemical processes.