We report a first study of the depolarization behavior of spin polarized 3 He in a mixture of 3 He-4 He at a temperature below the 4 He λ point in a deuterated TetraPhenyl Butadiene-doped deuterated PolyStyrene (dTPB-dPS) coated acrylic cell. In our experiment the measured 3 He relaxation time is due to the convolution of the 3 He longitudinal relaxation time, T1, and the diffusion time constant of 3 He in superfluid 4 He since depolarization takes place on the walls. We have obtained a 3 He relaxation time of ∼ 3000 seconds at a temperature around 1.9 K. We have shown that it's possible to achieve values of wall depolarization probability on the order of (1 − 2) × 10 −7 for polarized 3 He in the superfluid 4 He from a dTPB-dPS coated acrylic surface. Hyper-polarized 3 He based on the technique of optical pumping [1,2] has found applications in diverse fields such as in the study of quantum phenomena in low temperature fluids [3] and in the search for violations of fundamental symmetries [4,5,6]. They are also routinely used as polarized neutron spin-filters [7], as effective polarized neutron targets for nuclear and particle physics experiments [8] and for low magnetic field magnetic resonance imaging [9]. All such applications have motivated, as well as benefited from, studies of the relaxation mechanisms of polarized 3 He in gas, liquid and superfluid phases and under different surface conditions. The study of relaxation of polarized 3 He in 3 He -4 He mixtures at low temperatures is, however, of longstanding interest in its own right [10]. The simple atomic structure of 3 He makes a system of 3 He atoms ideal to model correlated fermions.Super-thermal production of Ultra-Cold Neutrons (UCN) from superfluid 4 He has been demonstrated [11] as an efficient way of producing a large number of UCNs. The capability of storing a large number of UCNs following their production is important for experiments studying fundamental properties of the neutron, for example the experiment on the search of the neutron electric dipole moment [6]. In this experiment, the deuterated TetraPhenyl Butadiene-doped deuterated PolyStyrene (dTPB-dPS) coated acrylic surface is chosen for such an application because of the small neutron absorption rate on the surface and its wavelength shifting property. The focus of this work is a first study of polarized 3 He relaxation time in a mixture of 3 He-4 He below the 4 He λ point in a dTPB-dPS coated acrylic cell. Such a study may find applications in the development of cryogenic 3 He magnetometers for experiments where trapping of polarized UCNs is involved as well as in other types of applications where polarized 3 He atoms are employed at low temperatures. At present the feasibility of 3 He magnetometers for UCNs has been studied only at room temperature [12].While a number of experiments [15,16,17,18,19] have reported 3 He longitudinal relaxation times (T 1 ) in mixtures of 3 He-4 He at temperatures similar to our work, the measurements most relevant to ours are [16,18,19]