Methods for determining the chemical structures, substructures and/or ionic structural arrangements of unknown or partially structurally characterized compounds from mass spectrometric data are disclosed. The methods of the invention are directed to elucidating the molecular structures of compounds by identifying the structures of ions generated from those compounds through the use of tandem mass spectrometry and subsequently conducting a library search of structurally characterized tandem product spectra from known ions for specific matches. The collective structural information and/or alternative structural candidates are then constructed from the determined ion structures and other structural characteristics using a structure generator and/or a structure assembly and/or a structure reduction system. The structure or substructure determination process can be continuously improved by adding additional data, e.g., information obtained from molecules previously structurally characterized using this method.