In a series of experiments we have found that the field in the ZnS layer of an ac-operated thin-film high-field electroluminescent device becomes clamped at values ranging from 2.1×106 to 3×106 V/cm. The device is fabricated as a sandwich with the following layers from top to bottom: thin metal; ZnS:Tb3+; Ta1.98Mo0.02O5 and Ta0.99Mo0.01. Since the electroluminescence is expected to be very strongly field dependent because the excitation proceeds via direct impact of the Tb3+ ions by hot carriers, the observed field clamping may impose a serious limitation on the efficiency that one can obtain from such structures. Evidence for field clamping is obtained by measuring as a function of peak-to-peak applied voltage the following: (i) Tb3+ emission intensity, (ii) the ratio of emissions originating in two different Tb3+ levels and (iii) the magnitude of a pulsed photocurrent. In addition, the clamped field may be determined quantitatively through the use of a novel technique which permits the measurements of the current-field relationship in the active layer. The value of the clamped field depends only slightly on the method of preparation of the ZnS layer and is insensitive to the type of metal contact used, to the Tb concentration, and to temperature. Several mechanisms which could give rise to the steep current-field relationship responsible for field clamping are considered. One of these, multiplication by impact ionization, is ruled out by the pulsed photocurrent measurements; Frenkel-Poole emission and Schottky emission are too temperature dependent and do not exhibit sufficiently steep current-field characteristics. It is concluded that field emission at the contacts must be responsible although it is not entirely clear why it is so insensitive to the contact materials.