A phosphorus-containing silsesquioxane is represented by the formula [R13SiO1/2]m[R2SiO3/2]n[R3SiO3/2]p[(R4O)2PO(CH2)xSiO3/2]q wherein each of R1, R2, R3, and R4 independently represents a hydrocarbyl group; x represents an integer of from 1 to 8; m is a positive number less than 1.5; n and q are positive numbers greater than 0 and less than 1; and p is a number greater than or equal to 0 and less than 1. Further, (n+p)/q is in a range of from 0.5 to 99, and (n+p+q)=1. Curable and cured compositions comprising the phosphorus-containing silsesquioxane are disclosed.