[en] The electron swarm parameters of R1225ye(Z) (cis--pentafluoroprop-1-ene) and its mixtures with the buffer gases N₂ and CO₂ (content of R1225ye(Z) from 1% to 50%) are experimentally investigated using a pulsed Townsend experiment. The analysis of the electron avalanche displacement current yields the effective ionization rate coefficient, the electron drift velocity and the longitudinal electron diffusion coefficient mainly around the density-reduced critical electric field , which is of interest for application in medium or high voltage insulation. In pure R1225ye(Z), increases with increasing pressure from around 280 Td at 2 kPa to 300 Td at 14 kPa. We assume that this is due to a three-body attachment rate coefficient of approximately . Mixtures of R1225ye(Z) with N₂ and CO₂ are discussed and a comparison to the structurally similar gases HFO1234ze(E) and l-C₃F₆ is given, which also exhibit three-body attachment. (paper)