An applied magnetic field is known to produce novel effects in the phase behavior of magnetoelastic spin-Peierls systems. Hence we report measurements of the differential susceptibility χ and magnetization M in fields up to 40 kOe (4T) on the spin-Peierls compound TTF-AuS4C4(CF3)4 in the temperature region 1.1 K⩽T⩽4.2 K. The range of field and temperature encompasses an interesting phase region, including the zero-field spin-Peierls transition temperature TC(0) = 2.03 K. The measurements of the differential ac susceptibility provide a more sensitive probe of the transition behavior than magnetization measurements. The first definitive evidence for significant deviations from mean-field critical behavior appear in these measurements, and the appropriate criteria for determining the precise location of the transitions are thus provided by the thermodynamic theory of λ transitions. Using the new criteria, qualitative and even quantitative agreement is obtained with current theories of the field dependence of spin-Peierls transitions. A novel contour plot of χac in the H-T plane is shown to be useful for the delineation of the global phase transition behavior. An investigation of the role of relaxation effects in χac relative to the nature of the phase boundaries is conducted. A major feature is the observation of a striking degree of ’’universality’’ in the phase behavior of three spin-Peierls systems, TTF-AuS4C4(CF3)4, TTF-CuS4C4(CF3)4, and MEM-(TCNQ)2. These universal features are preserved through considerable differences in lattice structure and a variation in TC (0) of a factor of 10. A complete report on this work has been submitted to Physical Review B. a)Supported in part at URI by NSF and at GE by AFOSR. bOn sabbatical leave from URI, now returned there.