Unsymmetrical and generalized indirect covariance processing methods provide a means of mathematically combining pairs of 2D NMR spectra that share a common frequency domain to facilitate the extraction of correlation information. Previous reports have focused on the combination of HSQC spectra with 1,1-, 1,n-, and inverted 1JCC 1,n-ADEQUATE spectra to afford carbon–carbon correlation spectra that allow the extraction of direct (1JCC), long-range (nJCC, where n ≥ 2), and 1JCC-edited long-range correlation data, respectively. Covariance processing of HMBC and 1,1-ADEQUATE spectra has also recently been reported, allowing convenient, high-sensitivity access to nJCC correlation data equivalent to the much lower sensitivity n,1-ADEQUATE experiment. Furthermore, HMBC-1,1-ADEQUATE correlations are observed in the F1 frequency domain at the intrinsic chemical shift of the 13C resonance in question rather than at the double-quantum frequency of the pair of correlated carbons, as visualized by the n,1, and m,n-ADEQUATE experiments, greatly simplifying data interpretation. In an extension of previous work, the covariance processing of HMBC and 1,n-ADEQUATE spectra is now reported. The resulting HMBC-1,n-ADEQUATE spectrum affords long-range carbon–carbon correlation data equivalent to the very low sensitivity m,n-ADEQUATE experiment. In addition to the significantly higher sensitivity of the covariance calculated spectrum, correlations in the HMBC-1,n-ADEQUATE spectrum are again detected at the intrinsic 13C chemical shifts of the correlated carbons rather than at the double-quantum frequency of the pair of correlated carbons. HMBC-1,n-ADEQUATE spectra can provide correlations ranging from diagonal (0JCC or diagonal correlations) to 4JCC under normal circumstances to as much as 6JCC in rare instances. The experiment affords the potential means of establishing the structures of severely proton-deficient molecules. Copyright © 2013 John Wiley & Sons, Ltd.