NPH-II is a prototypical member of the DExH/D subgroup of superfamily II helicases. It exhibits robust RNA helicase activity, and a detailed kinetic framework for unwinding has been established. However, like most SF2 helicases, there is little known about its mode of substrate recognition and its ability to differentiate between RNA and DNA substrates. Here, we employ a series of chimeric RNA-DNA substrates to explore the molecular determinants for NPH-II specificity on RNA and to determine if there are conditions under which DNA is a substrate. We show that efficient RNA helicase activity depends exclusively on ribose moieties in the loading strand and in a specific section of the 3'-overhang. However, we also document the presence of trace activity on DNA polymers, showing that DNA can be unwound under extremely permissive conditions that favor electrostatic binding. Thus, while polymer-specific SF2 helicases control substrate recognition through specific interactions with the loading strand, alternative specificities can arise under appropriate reaction conditions.