The invention relates to a process for preparing an ester mixture, in which an n-butene-containing feed mixture having a composition which changes over time is first oligomerized and then converted by hydroformylation, hydrogenation and esterification to an ester mixture. In this process, an approximation of the actual viscosity of the ester mixture is determined. The problem that it addresses is that of specifying a comparatively simple process which enables conversion of an n-butene with a variable composition over time to an ester mixture having a viscosity which can be kept very substantially constant over a long period even when an inconstant C4 source which delivers fluctuating qualities over this period is utilized. This is achieved through controlled use of a second raw material, namely ethene. It has been found that the viscosity of n-butene-based ester mixtures can be influenced by controlled use of ethene in the preparation of the ester precursors. Specifically, the invention proposes two measures by which the ethene can be used: either directly as C2 olefin or as C8 olefin after prior separate oligomerization.