Let F denote the finite field of order q For f ( x ) in F [ x ] , let f * ( x , y ) denote the substitution polynomial f ( x ) − f ( y ) . The polynomial f * ( x , y ) has frequently been used in questions on the values set of f ( x ) In this paper we consider the irreducible factors of f * ( x , y ) that are “solvable by radicals” We show that if R ( x , y ) denotes the product of all the irreducible factors of f * ( x , y ) that are solvable by radicals, then R ( x , y ) = g ( x ) − g ( y ) and f ( x ) = G ( g ( x ) ) for some polynomials g ( x ) and G ( x ) in F [ x ] .