By employing thermal expansion measurement, the objectives of this thesis are to understand the unusual phase transition mechanism of Gd5(Si[Subscript x]Ge[Subscript 1-x])4, the effect of impurities in starting material Gd on the its Curie point transition, etc. The main results are contained in two published papers. In the first paper, measurements of thermal expansion of single crystal Gd5(Si195̣Ge205̣) during cooling and heating were conducted for the first time. A very steep change in strain with temperature was observed when the material underwent a phase transformation. This was an unusual simultaneous magnetic and structural phase transformation from a ferromagnet with orthorhombic crystal structure below the transition temperature T[Subscript c] to a paramagnet with monoclinic crystal structure above T[Subscript c]. This transition temperature T[Subscript c] was found to depend on magnetic field, and to exhibit hysteresis depending on whether the material was being cooled or heated. When the material was subjected to a magnetic induction B in the range 0 - 2.5 Tesla, the transition temperatures, on both cooling and heating, were found to increase linearly with temperature by about 4.9 K/Tesla. This rate of change of transition temperature with magnetic field was in good agreement with calculations based on the assumption that the additional energy due to the magnetic field can suppress the thermal vibration of Gd atoms and that the additional thermal energy per Gd atom needed to cause the phase transition to occur is equal to the additional magnetic energy of each Gd atom caused by the magnetic field. In the second paper, two polycrystalline Gd5(Si209̣Ge191̣) samples were made by using high purity Gd and commercial Gd respectively but with Si and Ge starting materials of the same purity. Thermal expansion results showed that both samples exhibited a first order phase transformation. Magnetic force microscopy has been used to demonstrate the magnetic phase transformation process from paramagnetic to ferromagnetic upon cooling. It was found that the Curie temperature was lower and the thermally-induced strain higher in the sample made from lower purity Gd starting materials compared with the sample made from high purity Gd metal.