Phosinaite-(Ce), ideally Na 13 Ca 2 REE[Si 4 O 12 ](PO 4 ) 4 , has been found in sodalite syenite xenoliths at the Poudrette Quarry, Mont Saint Hilaire, Quebec. The mineral occurs as colorless to pale brown anhedral, elongate grains up to 2 mm set in the groundmass, and euhedral prismatic crystals up to 2X1 mm in vugs in sodalite. Crystals commonly are coated in a brown film of an amorphous Na-Ca-K silicate, previously believed to be a hydrocarbon. Important associated phases include microcline, analcime, albite, villiaumite, astrophyllite, thermonatrite, vitusite, revdite and vuonnemite. Average results of electron-microproble analyses (n = 4) gave Na 2 O 31.68, CaO 8.92, MnO 0.89, La 2 O 3 4.18, Ce 2 O 3 6.52, Nd 2 O 3 0.99, Pr 2 O 3 0.26, ThO 2 1.84, Al 2 O 3 0.11, SiO 2 18.36 and P 2 O 5 23.69 wt.%, corresponding to (Na (sub 12.72) Ca (sub 0.14) ) (sub Sigma 12.86) (Ca (sub 1.84) Mn (sub 0.16) ) (sub Sigma 2.00) (Ce (sub 0.49) La (sub 0.32) Th (sub 0.09) Nd (sub 0.07) Pr (sub 0.02) ) (sub Sigma 0.991 [(Si (sub 3.77) P (sub 0.15) Al (sub 0.04) ) (sub Sigma 3.96) O 12 ] (PO 4 ) 4 , based on 28 oxygen atoms. Phosinaite-(Ce) from Mont Saint-Hilaire is found to have a distinctive concentration of Th compared to that from the Khibina and Lovozero massifs, Russia. Single-crystal study showed it to be orthorhombic, P22 1 2 1 , with a 12.297(2), b 14.660(3), e 7.245(1) Aa, V 1306.1(1) Aa 3 and Z = 2. The observed powder-pattern is consistent with that previously published; the reflection reported at 6.92 Aa was not observed in our material. The crystal structure was refined using single-crystal X-ray-diffraction data to R = 3.1% and wR 2 = 7.99%. In addition to confirmation of the basic features of the structure originally proposed for the mineral, site-occupancy refinement for one octahedrally coordinated site gave (Na (sub 0.66) Ca (sub 0.34) ). A high degree of order of Si and P was established on the basis of results from electron-microprobe analyses, site-occupancy refinement, mean T-O bond distances and calculated bond-valence sums, suggesting that the mineral be classified as a silicophosphate.