The magnetoresistance oscillations, which occur in a two-dimensional electron system exposed to strong microwave radiation when the microwave frequency ω coincides with the n-th subharmonic of the cyclotron frequency ωc have been investigated for n = 2, 3 and 4. It is shown that these subharmonic features can be explained within a non-equilibrium energy distribution function picture without invoking multi-photon absorption processes. The existence of a frequency threshold above which such oscillations disappear lends further support to this explanation. PACS numbers: 72.20.Fr, 72.20.My, 73.40.Kp The observation of microwave induced magnetoresistance oscillations (MIMO) with some minima saturating near zero in state-of-the-art two-dimensional electron systems [1,2,3] has attracted great interest. The effect is governed by the ratio of the microwave frequency to the cyclotron frequency: ω/ω c . At the magnetic field close to an integer ratio, the magnetoresistance remains unaffected by the microwave radiation (zero-response node). At slightly larger values of ω/ω c , the microwaves cause a drop of resistance, while at smaller values they produce a positive contribution to the resistance. The two mainstream approaches to explain these oscillations are based on scattering assisted indirect optical transitions [4,5] and on the creation of a non-equilibrium electron energy distribution function [6,7]. Already in early experiments [2, 6, 8], additional oscillations were detected near ω = ω c /2 as well as close to other fractional values of ω/ω c such as 3/2, 5/2, and 2/3. In a recent article [9], these 'fractional' oscillations were tentatively ascribed to multi-photon processes involving the simultaneous absorption of n photons. Here, n = 2 or 3 and corresponds to the denominator of the fractional value. This interpretation was supported on a qualitative level within the model of microwave induced indirect transitions when multi-photon absorption is included [10,11,12].In this paper, we report on the observation of MIMO when ω = ω c /n with n = 2, 3 and 4 and in particular we investigate the frequency range where these additional oscillations are observed. We find that they only occur at microwave frequencies below a certain threshold value. If interpreted in terms of indirect optical transitions our data would imply the involvement of two-, three-and four-photon absorption processes. However within this framework, it is difficult to account for the frequency threshold. We show that all important properties of these additional oscillations, including the frequency threshold, are captured by the non-equilibrium electron distribution function picture when only single photon absorption processes are considered.Our investigations were carried on a double-sided modulation doped GaAs/AlGaAs quantum well with an elec-tron density of n s = 2.7 × 10 11 cm −2 and a mobility µ = 17 × 10 6 cm 2 /V s. Hall bar geometries were prepared with a width W of 400 µm and with potential probes spaced a distance L = 800 µm apart. The sam...