Poly(bismethoxyphosphazene) (PBMP) was synthesized, giving polymer melts with T-g = -70.9 degrees C. The chain dynamics of melts and DMF solutions of poly(bismethoxyphosphazene) (PBMP) was investigated by Small-Angle Neutron Scattering (SANS). The radius of gyration R-g was found as 144.7 +/- 0.2 angstrom, and the average MW from SANS was 95000 Da. DMF revealed to be a good solvent for this polymer. From the scattering intensity of polymer solutions for high q-data, the slope was -1.647 in almost exact agreement with the expected excluded volume exponent by Flory which is (5)/(3) affirming the good solvent property of DMF. GPC measurements of THF solutions were evaluated based on a universal calibration of polystyrene standards resulting in a rather similar value of 1.05 x 10(5) Da. Another series of SANS experiments was done with solutions of LiSO3CF3 (LiTf) in the title polymer. They also showed low T-g values down to -50 degrees C at 15 wt %. The SANS results (5 and 10 mol % LiTf as referred to the monomer units) showed almost no effect of the dissolved salt on the melt conformation of the polyphosphazene (almost random coil), and thus revealed a rather small interaction between salt and polymer. We also measured the ionic conductivity of salt-in-polymer systems with concentrations from 5 to 20 wt % LiTf. The room-temperature conductivity was 1.7 x 10(-5) S/cm at 20 wt % LiTf and is thus rather high. The low interaction between salt and polymeric solvent,is in agreement with the predominance of neutral ion pairs which is often observed in such polymer electrolytes.