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 &khgr; and magnetizationMin fields up to 40 kOe (4T) on the spin‐Peierls compound TTF‐AuS4C4(CF3)4in 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 temperatureTC(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 &lgr; 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 &khgr;acin 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 &khgr;acrelative 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 inTC(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.