The type of romarchite and hydroromarchite from Boundary Falls, Winnipeg River, Ontario, Canada, was re-examined. The minerals form deposits on pewter bowls and are products of tin corrosion in a unique cold freshwater environment. Type romarchite is very fine grained and replaces pseudotetragonal crystals of an unknown precursor. Revised X-ray powder diffraction data are consistent with the accepted tetragonal symmatry and space group P4/nmm, and provide the refined unit-cell parameters a 3.8014(3), c 4.8559(6) Ã…. Type hydroromarchite forms euhedral single crystals. Revised X-ray powder-diffraction data are consistent with the accepted tetragonal symmatry and space group P421c, and provide the refined unit-cell parameters a 7.9259(5), c 9.0970(9) Ã…. Additional Sn-bearing corrosion products are SnO2 (cassiterite) and two unidentified phases. Residual aggregates of Pb, SbSn and Cu6Sn5 (?-bronze) are found in areas of deep localized corrosion. Reports of romarchite subsequent to the original descriptions are numerous, but those of hydroromarchite are few and only quite recent.