In this work, a new heterogeneous catalyst for the biodiesel synthesis was produced by the combination of chrysotile with KOH and thermal treatment. SEM, XRD, TG/DTA, BET, titration and CO2-TPD results suggest that chrysotile fibers [Mg3Si2O5(OH)(4)] impregnated with KOH decompose at 700 degrees C to produce safer sintered large particles containing a catalytically active K+ doped MgO dispersed phase. This K+/MgO phase showed surface basic sites with very good activity for the transesterification of soybean oil with methanol under different conditions. Control experiments suggest that K+ is not leached by methanol and the catalyst can be reused for several times. (C) 2013 Elsevier B.V. All rights reserved.