This article draws on data obtained through research on pronunciation of loanwords among Czech speakers in the two most populous cities in the Czech Republic (Prague, Brno) in 2014. More than 60 words out of the total number of 300 expressions included in the survey were investigated from the perspective of fluctuation in voicing. Most of these words consist of Anglicisms and lexemes of Latin or Greek origin. The pairs s/z and k/g fluctuate most often; several other pairs also display some fluctuation peripherally. As concerns the pair s/z, in accordance with previous studies, it can be stated that in contemporary Czech, the preference for the voiced variant prevails. It cannot be decidedly stated, however, that voicing will spread further and that it will prevail in all cases in the future; the resultant forms are actually influenced, for example, by the language of origin, the time period of the borrowing, the distribution of the word, the type of fluctuating consonants, or the neighbouring sounds. As concerns sociolinguistic categories, age and education have proven to be especially important. For example, in some recently borrowed Anglicisms, the voicing will probably continue to prevail strongly, and on the contrary, it will likely tend to recede in the groups kr, kl in expressions adapted earlier. The fluctuation of voicing in loanwords and proper names is natural; if it does not influence the comprehensibility or the quality of the speech, it does not cause any communication problems.