The effect of particles on the critical strain, epsilon(c), associated with the Portevin-LeChatelier (PL) effect of aluminium alloys is studied using Al-Mg-Ni and Al-Si alloys. Al-Mg-Ni and Al-Si alloy matrixes are composed of Al3Ni and Si particles, respectively. Tensile tests were performed in the temperature range 223-273 K in which the critical strain decreases with increasing temperature, and strain rates between 10(-5) and 10(-2) s(-1) were chosen. According to the apparent activation energies, Q, Mg and Si solute atoms are responsible for the flow instability in Al-Mg-Ni and Al-Si alloys, respectively. The experimental results also show that the critical strain decreases with decreasing particle spacing, d(p). Since the particle spacing is small compared to the corresponding grain size, the decrease in critical strain should be ascribed to the effect of particles. Considering that the dislocation density is increased by the particles, a modified model showing the critical strain, epsilon(c), as a function of particle spacing, d(p), is proposed as epsilon proportional to epsilon(c)(beta(gamma+1/2)) d(p)(-n(gamma+1/2)) T-1 exp (-Q/kT), in which epsilon, T and k are the strain rate, temperature and Boltzmann constant, respectively. Linear fit of the plots of In epsilon(c) versus In d(p) and In epsilon versus In d(p) indicates that this equation is appropriate to rationalize the particle effect on the critical strain.