Single crystals of α-methyl alaninium dihydrogen monophosphate (a-MADP) and α-methyl alaninium dihydrogen monoarsenate (α-MADA) were grown from aqueous solutions by a controlled isothermal evaporation method at 315 K. The chemical compositions of both compounds resulted from an automatic elementary analysis. They are consistent with the empirical formula (CH 3 ) 2 C(NH 2 )COOH · H 3 P0 4 (a-MADP) and (CH 3 ) 2 -C(NH 2 )COOH · H 3 As0 4 (α-MADA). At room temperature the crystals belong to the triclinic space group P\ with cell parameters: a = 10.086 Â, ¿> = 7.641 A, c = 5.871 Â, α = 97.7°, β= 108.0°, 7 = 81.0° (α-MADP) and α = 10.164 À, ¿> = 7.830 Â, c = 5.935 Â, α = 97.2°, 0=107.7°, y = 81.9° (α-MADA). The magnitudes and the directions of the 3 principal refractive indices were measured at 589 nm and 295 K. Caloric measurements revealed phase transitions at 215 Κ (α-MADP) and 178 Κ (α-MADA), which could be detected also by dielectric methods. The principal values of the dielectric constant of α-MADP and α-MADA were determined for different temperatures below and above the phase transition. Dielectric hysteresis measurements with fields up to 20 kV/cm gave no indication of a ferroelectric, antiferroelectric or nonlinear behaviour.

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