This paper has two aims: (i) to introduce a novel method for measuring which part of overall citation inequality can be attributed to differences in citation practices across scientific fields, and (ii) to implement an empirical strategy for making meaningful comparisons between the number of citations received by articles in the 22 broad fields distinguished by Thomson Scientific. The paper is based on a model in which the number of citations received by any article is a function of the article’s scientific influence, and the field to which it belongs. The model includes a key assumption according to which articles in the same quantile of any field citation distribution have the same degree of citation impact in their respective field. Using a dataset of 4.4 million articles published in 1998-2003 with a five-year citation window, we find that differences in citation practices between the 22 fields account for about 14% of overall citation inequality. Our empirical strategy for making comparisons of citation counts across fields is based on the strong similarities found in the behavior of citation distributions over a large quantile interval. We obtain three main results. Firstly, we provide a set of exchange rates to express citations in any field into citations in the all-fields case. (This can be done for articles in the interval between, approximately, the 71st and the 99th percentiles of their citation distributions). The answer is very satisfactory for 20 out of 22 fields. Secondly, when we normalize the raw citation data with our exchange rates, the effect of differences in citation practices is reduced to, approximately, 2% of overall citation inequality in the normalized citation distributions. Thirdly, we provide an empirical explanation of why the usual normalization procedure based on the fields’ mean citation rates is found to be equally successfully.