A theory describing the effects of a DC magnetic field on a (3) Sigma(+)-A(1) Pi complex includes the influence on the energy positions of the (3) Sigma(+) rovibrational levels, the change of their lifetimes as well as the strength of the (3) Sigma(+)<--(1) Sigma(+) intercombination transitions. This is measured on the a＇(3) Sigma(+)(v=14)<--X-1 Sigma(+) (v=0) transition in CO where strong zero field mixing of the a＇(3) Sigma(+)(v=14) state with the A(1) Pi(v=4) state occurs through spin-orbit coupling. a＇<--X excitation spectra are taken under fields of up to 1 Tesla. Lifetimes of various a＇(v=14),N,F-i rotational levels are measured as a function of a magnetic field, where changes as large as 30% reveal a strong influence of the field on the mixing with the A(1) Pi(v=4) state. A change in the singlet character of the a＇(3) Sigma(+)(v=14),N,F-i rovibrational levels alters the line intensities of their forbidden a＇<--X transition which arises due to intensity borrowing from the allowed A<--X transition. From the measured lifetime values at 1 Tesla the changes in line intensities as well as absolute oscillator strengths for several a＇(v=14)<--X(v=0) rotational lines are derived. An increase in population transfer from X-1 Sigma(+)(v=0),J to a＇(3) Sigma(+)(v=14),N,F-i of up to 51% is deduced. Finally, the energy levels and the lifetimes of the a＇(v=14),N,F-i rotational levels in the high field regime beyond 4 Tesla are discussed.