ASH-FREE detergent additives of the succinimide type, which have considerable solubilizing action, have recently at tracted general attention [1, 2]. Many additive compositions intended for making high duty motor oils also have a solubilizing effect [3]. As shown by previous studies [2, 3] not only detergent additives, but many other additives are solubilizing agents, metal dithiophosphates in particular, which are mainly used as anti-oxidation components. On combining some detergent and anti-oxidation additives a synergistic effect is observed, which markedly increases the solubilizing effect of the additive composition compared with each component separately [4]. I t should be noted that enough attention is not always given to the selection of concentration of additives used with oils, particularly when making up compositions of many components; ill many cases additive concentration increases consequently, certain components conflict when used in a form mixed with each other. Considering the importance of the solubhizing action of modern additives used in motor oils in ensuring a considerable detergent dispersion effect, we investigated the influence of additive concentration on the solubilization of insoluble products. The solubilizing ability of additives was determined by methods previously described [2, 5]. This method is based on the fundamental principle that the dye rhodamine C and asphaltenes are practically insoluble in iso-octane, but on adding substances to iso-octane, which have a solubilizing effect, rhodamine C and asphaltenes are dissolved and give colour to the solution. Thus, the solubilization of insoluble products can be evaluated from the variation on light absorption by iso-octane before and after the interaction of the dye or asphaltenes with additives. Detergent and anti-oxidation additives of different structures, some ash-free and others containing metals, were selected for investigation. The study of the behaviour of succinimides, which are necessary components of high-gTade motor oils was of particular interest [1, 6]. The firs~ series of experiments were carried out to determine the critical