We have studied the eﬀect of unsupported Pd nanoparticle (NP) size in the selective CC semi-hydrogenation of alkynols with diﬀerent alkyl chains, i.e., C16 in dehydroisophytol (DIP) (to isophytol (IP)) vs C1 in 2-methyl3-butyn-2-ol (MBY) (to 2-methyl-3-buten-2-ol (MBE)). The Pd NPs were synthesized via colloidal technique with poly(N-vinyl2-pyrrolidone) (PVP) as stabilizing agent where a range of crystal sizes (2.1−9.8 nm; conﬁrmed by HRTEM) was generated. Both reactions show antipathetic structure sensitivity consistent with higher speciﬁc activity (TOF) over larger Pd NPs where the structure sensitivity eﬀect is more pronounced for NPs ≤ 3.0 nm. All the Pd NPs exhibit high (≥88%) selectivity to the target alkenol product at almost complete (98%) conversion. Increased IP selectivity (SIP; XDIP=98% ca. 95%) was observed over smaller (2.1−3.0 nm) Pd NPs while ca. 98% selectivity to MBE (SMBE; XDIP=98%) is obtained irrespective of particle size. The kinetic results were consistent with a Langmuir− Hinshelwood model. The observed Pd NPs size eﬀect on catalytic response is ascribed to a contribution of Pd electronic surface modiﬁcations, fraction of Pdplane active sites and the steric eﬀects which impact akynol/alkenol adsorption constants. The results obtained in this work provide a powerful tool for catalyst design for industrial applications.