For a ring R, Hilbert's Tenth Problem HT P (R) is the set of polynomial equations over R, in several variables, with solutions in R. We view HT P as an operator, mapping each set W of prime numbers to HT P (Z[W −1 ]), which is naturally viewed as a set of polynomials in Z[X 1 , X 2 , . . .]. For W = ∅, it is a famous result of Matiyasevich, Davis, Putnam, and Robinson that the jump ∅ ′ is Turing-equivalent to HT P (Z). More generally, HT P (Z[W −1 ]) is always Turing-reducible to W ′ , but not necessarily equivalent. We show here that the situation with W = ∅ is anomalous: for almost all W , the jump W ′ is not diophantine in HT P (Z[W −1 ]). We also show that the HT P operator does not preserve Turing equivalence: even for complementary sets U and U , HT P (Z[U −1 ]) and HT P (Z[U −1 ]) can differ by a full jump. Strikingly, reversals are also possible, with V < T W but HT P (Z[W −1 ]) < T HT P (Z[V −1 ]).