The viscoelastic instability condition (see section 2.8 in [1], see also [37]) is given by That is, electric fields give rise to extensional flows [3], with significant coil stretching, when De > 1 / 2 [1, 37] for dilute solutions; note that for melts a threshold of 0.5 is also reported, which indicates that this approximate threshold value may be universal [38]. For the HMW sample we calculate V$_{cr}$ ≈ 0 . 015 kV for our system [53]. Above this threshold instability is observed, however below ≈ 0.05 kV bias it becomes difficult to obtain data, due to diffusion rates becoming non-negligible for the low flow speeds, and the fluctuation in signal approaching the noise floor. To better observe the transition to instability the LMW sample is used at a concentra- tion of 120 ppm. The predicted critical driving voltage for the LMW sample is over an order of magnitude larger, V$_{cr}$ ≈ 0 . 28 kV (corresponding to a critical velocity of roughly 2 mm/s), than for the HMW sample. The threshold observed in Fig. 7, which occurs near the predicted value, supports the hypothesis that polymer stretching drives the instability [54]. The discrepancy between the observed and predicted threshold we attribute to both the relaxation time estimate and to existence of molecular weight distribution in polymer samples. Additionally, the theoretical critical Deborah number of 0.5 is approximate, and in direct experimental observation of DNA polymers in extensional flows [41] at a Debo- rah number of [55] ≈ 0.4 significant stretching occurred (the latter result would reduce the estimated V$_{cr}$ here to ≈ 0.23 kV, a velocity of 1.6 mm/s). Within the limited precision of predictions our experimentally observed instability threshold agrees well with theory. Two alternative instability mechanisms must be discussed. Conductivity gradients be- tween co-flowing streams can give rise to electrohydrodynamic (EHD) instabilities when driven by electric fields [42]. The possibility of EHD instability is ruled out, as the (oth- erwise identical) polymer free solutions do not show instability. It is also conceivable that dynamic polymer-wall adhesion [19] of polymer coils coating the walls could lead to a time varying µ$_{EOF}$ , driving instabilities. We reject this as underlying the instability, as PAAm does not adhere to glass [19]. This rejection is also supported by the similarity between µ$_{EOF}$ measurements of polymer-free and dilute (120 ppm) HMW polymer solutions which indicates no significant modification at the wall occurs. De = λv L$_{Char}$ ≃ λ$_{Zimm}$v L$_{Char}$ = λ$_{Zimm}$µ$_{EOF}$E L$_{Char}$ > 1 2 . (2) 13