The present invention relates to ethynyl derivatives of formula I wherein G, X, R1, R2, R3, R3', R4, and R4' are as defined herein or to a pharmaceutically acceptable acid addition salt, to a racemic mixture, or to its corresponding enantiomer and/or optical isomer and/or stereoisomer thereof. Compounds of formula I are positive allosteric modulators (PAM) of the metabotropic glutamate receptor subtype 5 (mGluR5).
