1. Introduction. Many diophantine problems can be reduced to (ordinary) unit equations and S-unit equations in two unknowns (for references, see e.g. [15] (with explicit constants). These led to a lot of applications.The purpose of the present paper is to considerably improve (in completely explicit form) the above-mentioned estimates in terms of the cardinality of S and of the parameters involved (degree, unit rank, regulator, class number) of the ground field. To obtain these improvements we use, among other things, some recent improvements of Waldschmidt [26] and Kunrui Yu [27] concerning linear forms in logarithms, some recent estimates of Brindza [5] and Hajdu [18] for fundamental systems of S-units, some upper and lower bounds for S-regulators (cf. Lemma 3 of this paper) and an idea of Schmidt [23]. Further, in our arguments we pay a particular attention to the dependence on the parameters in question. As a consequence of our result, we derive explicit bounds for the solutions of homogeneous linear equations of three terms in S-integers of bounded S-norm. These improve some earlier estimates of Győry [13], [14].An application of our improvements is given in [17] to decomposable form equations (including Thue equations, norm form equations and discriminant form equations) in S-integers of a number field. Some other applications will be published in two further works.