This work investigates the dielectric and barrier properties of two species of organic aromatic low dielectric constant (low-k) polymers, namely, FLARE and SiLK. Experimental results indicate that both of the low-k polymers exhibit acceptable thermal stability with respect to a thermal annealing at 400°C for 8 h in an N2 ambient. Moreover, they show a good dielectric barrier property against Cu penetration under bias-temperature stressing (BTS) at 150°C with an applied effective field of 0.8 MV/cm. Nevertheless, an anomalous instability of the capacitance-voltage curve was observed for the first time under BTS. This finding is explained by the proposed model of stress induced dielectric polarization charges within these organic aromatic polymers. The polarization instability may seriously degrade the long term reliability of circuit operations.