The variations of thermal expansion coefficient (TEC) and viscosity with refractive index modifier dopants have been measured. When PbF2 replaces BaF2 there is an increase in TEC accompanied by a decrease in both T5 and viscosity at fibre pulling temperature whereas when PbF2 replaces NaF there is a drop in TEC with a complementary increase in Tg and viscosity at fibre pulling temperature. Similarly when HfF4 replaces ZrF4 the TEC decreases while Tg and viscosity at fibre pulling temperature increase. The expression αTg2 = constant holds well for glasses not containing Li and allows the magnitude and direction of changes in TEC to be predicted from changes in Tg. The activation energy for viscous flow at small dopant levels varied little indicating that the magnitude and direction of viscosity changes can be predicted from Arrhenius behaviour and Tg differences. The stresses in a fibre resulting from expansion mismatch and those mechanically induced during drawing as a result of viscosity mismatch are small for common core clad structures. The thermal and mechanical stresses are generally of the same magnitude and opposing sign; they therefore do not enhance or degrade fibre strength.