Thermodynamic propensities of biomolecules to adopt non-native conformations are crucial for understanding how they function, but prove difficult to measure experimentally. Combining optical melting experiments with chemical modifications and mutations, we developed delta-Melt for measuring the energetic penalties associated with nucleic acid conformational rearrangements and how they vary with sequence and physiological conditions. delta-Melt is fast, simple, cost effective, and can characterize conformational penalties inaccessible to conventional biophysical methods.