This article aims to analyse the impact of transitional periods on researchers' mobility decisions in the enlarging Europe. A compilation of policy and law analyses, together with social research, is the adopted method for this study. Countries under research are Poland and Bulgaria, as sending countries, and Germany and the UK, representing the EU member states that opted for and against transitional periods, respectively. The political environment in Europe, in which researcher's mobility is taking place, is presented along with the characteristics of scientific mobility, as part of the international migratory flows. The question asked is, how the perception of enlargement influences mobility decisions of scientists. The results of qualitative research proved only to a certain extent the hypothesis, that countries with no transitional periods introduced were more attractive for scientists from new member states than the countries with such periods introduced. The analysis showed that although various administrative and legal barriers do represent impediments to scientific mobility, yet they were not usually decisive factors for mobility.