Nuclear magnetic resonance (NMR) of H-1 nuclei of liquids in saturated porous rocks furnishes an increasingly important tool in core analysis as well as in well-logging. In particular, there are several basic questions regarding NMR downhole logging in reservoir sandstones. For this reason, it is of interest to investigate the laboratory NMR properties to identify some important basic mechanisms. This paper presents new correlations among porosity, phi, irreducible water saturation, S-wi, formation resistivity factor, F, and the geometric mean longitudinal relaxation time, T-1g. Equilibrium magnetization and longitudinal relaxation, T-1, measurements were performed on 57 clean sandstone samples from 12 oil and gas reservoirs. NMR measurements phi(NMR) of phi agreed very well with conventional porosity measurements. The product of porosity and irreducible water saturation is approximately equal to an unfree fluid index, UFFI = (phi(NMR)/T-1g)(2/3), giving an error-of-estimate factor of 1.24. The best estimate of S-wi based on a simple cut-off relaxation time (35 ms) gave an error factor of 1.6. The present correlation of S-wi with phi(NMR) and T-1g is nearly as good as the correlation using T-1g and F, and it has the advantage of requiring only NMR measurements on fully saturated samples. No further improvement in correlation with S-wi was obtained by using T-1g with both phi and F over that with T-1g and F only. The correlation with F was slightly better than that with phi. The effectiveness of using both T-1 and either F or phi(NMR) is plausible, since T-1, being sensitive to surface-to-volume ratio, gives some information on pore or pore channel dimensions, while phi(NMR). being correlated with F, gives some information on connectivity within a sandstone. Large pores connected only by very small channels contribute to long T-1 but contribute also to Swi. The above use of phi(NMR) and T-1 should be useful for laboratory measurements on cores; furthermore, since T-2 is closely related to T-1 at the low measurement frequencies used in nuclear magnetism logging, it is very likely that a similar relationship (probably with different parameters) may relate S-wi to phi(NMR) and T-2 obtained in downhole measurements.