To understand the competition between magnetic ordering and superconductivity at low temperature in (Sn 1ุx Er x )(1)Er(2) 4 Rh 6 Sn(2) 18 compounds the variation as function x of the a.c. susceptibility and the magnetic structure, as determined by single-crystal neutron di4raction, were investigated. For increasing x, the low temperature properties change from coexistence of magnetic ordering and superconductivity (x โซุโฌ 0) to reentrant superconductivity (x+ +0.4) and to magnetic ordering only (x+ +0.6). The neutron di4raction experiments at low temperatures carried out for three samples of compositions x โซุโฌ 0, 0.42, and 0.6 allowed us to determine the variation of the low-temperature magnetic structure as a function of the mixed (Sn 1ุx Er x )(1) site composition. Our results indicate that the changes in the low-temperature physical properties are essentially related to the increase in coherence length of the magnetic ordering for increasing x. For x โซุโฌ 0, only short-range magnetic order of the Er(2) sublattice is present, with a correlation length of only a few unit cells large. For the intermediate x โซุโฌ 0.42 sample, superconductivity appears and is suppressed at lower temperature due to magnetic ordering. For x โซุโฌ 0.6, long-range magnetic order is established for one of the two Er sublattices, preventing the appearance of superconductivity. This allows the coexistence of superconductivity and short-range magnetic order.