Selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) chemistry allows local nucleotide flexibility to be quantitatively assessed at single nucleotide resolution in any RNA. SHAPE chemistry exploits structure-based gating of the nucleophilic reactivity of the ribose 2'-hydroxyl group by the extent to which a nucleotide is constrained or flexible. SHAPE chemistry was developed using N-methylisatoic anhydride (NMIA), which is only moderately electrophilic and requires tens of minutes to form ribose 2'-O-adducts. Here, we design and evaluate a significantly more useful, fast-acting, reagent for SHAPE chemistry. Introduction of a nitro group para to the reactive carbonyl to form 1-methyl-7-nitroisatoic anhydride (1M7) yields a reagent that both reacts significantly more rapidly with RNA to form 2'-O-adducts and is also more labile toward advantageous, self-limiting, hydrolysis. With 1M7, the single nucleotide resolution interrogation of the RNA structure is complete in 70 s. SHAPE analysis performed with 1M7 accurately reports the secondary and tertiary structure of the RNase P specificity domain and allows the secondary structure of this RNA to be predicted with up to 91% accuracy.