H,H and P,H coupling consts. and variable-temp. 1H and 13C NMR spectra and 13C T1 values for macrocyclic dithiophosphonic esters I (X = NH, OCH2CH2O) are presented to show that these compds. possess reduced flexibility in comparison with crown ethers. The NMR spectroscopic results are supported by an x-ray structure which shows a highly strained ring system.