A multivariate statistical method and a heuristic method were employed to examine the structural features associated with the persistence of degradation of 287 chemicals tested with the standard biochemical oxygen demand(BOD) procedure. The data base consisted of 196 'degradable' chemicals with a half-life of less than 15 d (e.g., Theoretical BODs (ThOD) > 16%) and 91 'persistent' chemicals with a half-life of more than 15 d. The multivariate statistical analysis consisted of (1) calculating 54 molecular connectivity indices, five physicochemical properties and eight principal components from the molecular connectivity indices, (2) clustering the chemicals on the basis of the principal components and one of the physicochemical properties, K(ow) and (3) discriminating between persistent and degradable chemicals using the molecular connectivity indices and the physicochemical properties as discriminating variables within each cluster. The heuristic approach used the results of the multivariate analyses and the literature on biodegradation to identify a series of structural features associated with degradable and persistent chemicals.