In this work, we consider the proportion of smooth (free of large prime factors) values of a binary form F (X1, X2) ∈ Z[X1, X2]. In a particular case, we give an asymptotic equivalent for this proportion which depends on F . This is related to Murphy's α function, which is known in the cryptographic community, but which has not been studied before from a mathematical point of view. Our result proves that, when α(F ) is small, F has a high proportion of smooth values. This has consequences on the first step, called polynomial selection, of the Number Field Sieve, the fastest algorithm of integer factorization.