Dynamic secondary-ion mass spectrometry (SIMS) was used to measure equilibrium volume-fraction profiles of a symmetric diblock copolymer at a polymer/polymer interface. The three polymers were a saturated polybutadiene with 90% 1,2-addition (sPB90) homopolymer, a saturated polybutadiene with 63% 1,2-addition (sPB63) homopolymer, and a sPB90-sPB63 diblock copolymer. The molecular weights of the homopolymers were chosen such that the sPB90/sPB63 blend was deep in the two-phase region. The experimentally determined volume-fraction profiles were compared with self-consistent-field theory calculations (SCFT). The statistical segment lengths and the Flory-Huggins interaction parameter were obtained independently from small-angle neutron scattering (SANS) measurements on a low molecular weight, homogeneous binary sPB90/sPB63 blend. Thus, no adjustable parameters were used in the SCFT calculations. Adsorption isotherms and the measured diblock copolymer volume-fraction profiles are in excellent agreement with the SCFT calculations. This work demonstrates that Flory-Huggins interaction parameters and statistical segment lengths determined by SANS can be used to predict complex phase behavior, including interfacial adsorption of block copolymers.