Earth Orientation Parameters (EOP) provide the rotation of the International Terrestrial Reference System (ITRS) to the Geocentric Celestial Reference System (GCRS) as a function of time. When estimating a Celestial Reference Frame (CRF) usually a number of radio sources with a long history of observations and stable positions are included in the datum used to define the orientation of the frame. How many and which radio sources are taken into account for the datum definition has a significant effect on the estimated EOP. In this study we analyze the effects of different options for the celestial datum definition on the precision of the EOP and on the agreement w.r.t the last realization of the International Celestial Reference Frame (ICRF2; Fey et al., The second realization of the international celestial reference frame by very long baseline interferometry, IERS Technical Note No. 35, 2009). The resulting EOP of the special VLBI session IYA09 are compared to the C04 08 EOP series (Bizouard and Gambis, The combined solution C04 for Earth orientation parameters consistent with international terrestrial reference frame 2008, IERS Notice 2011, 2011). The analysis shows that the smallest uncertainties for EOP are achieved when the maximum number of defining sources is chosen for the datum. Comparing with a typical VLBI session, the precision of the EOP and the agreement of the axes w.r.t. ICRF2 could be improved if more defining sources, especially in the southern hemisphere, were considered.