The goal of this paper is to explore in more detail the application of A / B testing in measuring the effectiveness of online sales. The aim of this paper was to determine exactly what changes to the user interface have the greatest impact on improving key performance indicators of online sales. As part of the project, an actual online controlled experiment was conducted in the form of A / B testing on a simple random sample of at least five thousand visitors in the experimental and control group. The experiment was conducted from January 2021 to July 2021. During the observed period, five A / B tests were conducted, four of which related to the manipulation of user interface elements, and one test examined the difference in key performance indicators depending on the quality of the search engine. The first A / B test shows the horizontal menu of the assortment category as a better solution compared to the drop-down menu. The results of the second A / B test show a personalized sliding bar with new products as a better solution compared to the sliding bar with manually selected new products. The results of the third test from this paper suggest the need for multiple testing of the same user interface element in order for the company’s online business to reach its full potential. By conducting the fourth A / B test, the company wanted to get an answer to the question of the impact of reviews other customers left on the purchase of future visitors. The results of this test show the dependence of the observed key performance indicators on the type of device used by the visitor. The last A / B test in this paper showed a positive correlation between the accuracy of search results and the growth of key performance indicators. Based on this test, a new search engine was implemented on the site as a targeted effort to raise the user experience to a higher level. Continuous implementation of A / B tests enables marketing professionals to understand the importance of certain elements of the user interface for the entire online business of the company. Despite many limitations, A / B testing is the simplest form of controlled online experiment, and using it, companies can easily get the data they need to optimize their online business.