The invention discloses core/shell type catalyst particles comprising a Mcore/Mshell structure with Mcore=inner particle core and Mshell=outer particle shell, wherein the medium diameter of the catalyst particle (dcore+shell) is in the range of 20 to 100 nm, preferably in the range of 20 to 50 nm. The thickness of the outer shell (tshell) is about 5 to 20% of the diameter of the inner particle core of said catalyst particle, preferably comprising at least 3 atomic layers. The core/shell type catalyst particles, particularly the particles comprising a Pt-based shell, reveal a high specific activity. The catalyst particles are preferably supported on suitable support materials such as carbon black and are used as electrocatalysts for fuel cells.