The inverse Schulze-Hardy rule was recently proposed based on experimental observations. This rule describes an interesting situation of the aggregation of charged colloidal particles in the presence of the multivalent coions. Specifically, it can be shown that the critical coagulation concentration is inversely proportional to the coion valence. Here the derivation of the inverse Schulze-Hardy rule based on purely theoretical grounds is presented. This derivation complements the classical Schulze-Hardy rule which describes the multivalent counterion systems.The aggregation of charged colloids is a long-studied phenomenon. More than 100 years ago Schulze and Hardy showed that the aggregation power of salts depends strongly on the ion valence [1,2]. More precisely, the critical coagulation concentration (CCC) (i.e., concentration of salt at which particles start to aggregate fast) [3] decreases very rapidly by increasing counterion valence. This discovery was later confirmed theoretically by Derjaguin, Landau, Verwey and Overbeek and is known as the DLVO theory [4,5]. They have shown that by assuming the interaction between particles as a sum of van der Walls (vdW) and double layer forces (DL) in the symmetric z:z electrolyte the CCC is inversely proportional to the sixth power of the valence,The above relation, also named Schulze-Hardy rule, is valid for highly charged particles. This explanation confirmed the DLVO theory and made it widely accepted. The symmetric z:z electrolytes are usually practically insoluble, therefore in experiments one typically uses asymmetric 1:z or z:1 multivalent electrolytes. In the case of asymmetric electrolytes, multivalent ions can either play a role of the counterions or the coions, where they have the opposite or the same charge as the colloidal particle, respectively. It was shown experimentally as well as theoretically that for highly charged particles the SchulzeHardy rule (1) is a good approximation also for asymmetric electrolytes where z is the counterion valence [6][7][8]. Recently Cao et al.[9] investigated a complementary problem, namely the influence of multivalent coions on the aggregation. In this situation experimental data could be reasoned with the inverse Schulze-Hardy rule, namelywhere z is the valence of the coion. Note that in the case of the coions the dependence on valence is much weaker. * E-mail: gregor.trefalt@unige.chInterestingly, in the low particle charge limit, where the DL forces can be described by the Debye-Hückel (DH) approximation, the samedependence for both counterions and coions is reached [8,9]. The latter low charge limit (3) lies between the Schulze-Hardy (1) and inverse Schulze-Hardy (2) dependences. This proposed inverse Schulze-Hardy rule therefore elegantly completes the understanding the aggregation in experimentally relevant asymmetric multivalent electrolytes. The Schulze-Hardy rule (1) and the low charge DH limit (3) were both derived theoretically. On the other hand, the inverse Schulze-Hardy rule was only given as an empirical ...