Novel compounds of Formula (I) <FORM:1095184/C2/1> in which each of R4 and R5 is a hydrogen atom or a C1- 5 alkyl or aryl-substituted C1- 5 alkyl group, or OR4 and OR5 are ortho to each other and R4 and R5 together represent methylene; M is a C1- 5 alkoxy, an aryl-substituted C1- 5 alkoxy, an amino, or a hydroxy group; and n is 0, 1 or 2, and acid addition salts thereof, are prepared from a compound of Formula (II) <FORM:1095184/C2/2> wherein R3 is acetyl or formyl; and each of R and R1 is a hydrogen atom, or a C1- 5 alkyl or aryl-substituted C1- 5 alkyl group, no more than one of R and R1 being hydrogen when OR and OR1 are ortho to one another, or R and R1, but only when OR and OR1 are ortho to one another, together represent a methylene radical or a <FORM:1095184/C2/3> radical, (1) by reaction of the nitrile with an alcoholic mineral acid, to produce the corresponding a  - hydroxymethyl - phenylalanine amide acid addition salt during which the borate complex, if present, is hydrolysed to give an ortho di-hydroxy compound, followed, if desired, (a) by reaction with a dilute aqueous solution of a mineral acid to produce the corresponding a -hydroxymethylphenylalanine acid addition salt, which may, if desired, be treated with an excess of concentrated aqueous hydrochloric or hydrobromic acid at reflux temperature to convert any alkoxy substituents on the benzene ring to hydroxy groups, or (b) by reaction with an excess of concentrated aqueous hydrochloric or hydrobromic acid at a temperature of 75-126 DEG  C. to produce the corresponding a -hydroxymethyl-(hydroxy or methylene-dioxy) phenylalanine hydrobromide or hydrochloride, (2) by reaction with a dilute aqueous solution of a mineral acid to produce the corresponding a -hydroxymethylphenylalanine acid addition salt during which the borate complex, if present, is hydrolysed to give an ortho di-hydroxy compound, followed, if desired, by the reaction described under (1b) above, or (3) by reaction with an excess of a concentrated aqueous hydrochloric or hydrobromic acid at a temperature of 75-126 DEG  C. to produce the corresponding a -hydroxymethyl-(hydroxy or methylenedioxy) phenylalanine hydrobromide or hydrochloride.  (During reaction (1b) and (3) any alkoxy groups are dealkylated and the borate complex is hydrolysed, but any methylenedioxy groups are unaffected). The a -hydroxymethylphenylalanine acid addition salts thus produced may be converted to the free bases by reaction with a lower alkylene oxide or an inorganic base in an inert solvent. The esters may be made by treating the free base or an acid addition salt with an alcohol in the presence of a mineral acid at an elevated temperature.  The starting material of Formula II may be prepared by a process as claimed in Specification 1,095,187.  The a -hydroxymethyl-(hydroxy) phenylalanine amide acid addition salt may also be prepared by hydrolysis of the corresponding 2 - (acetamido or formamido)-2-(benzyloxybenzyl) - 3 - hydroxypropionitrile with an alcoholic mineral acid (reaction (1) above), followed by reduction with hydrogen using a palladium on charcoal catalyst.ALSO:Therapeutic compositions which are active as inhibitors of mammalian decarboxylase and are valuable anti-hypertensive agents, and which may be administered orally or parenterally, contain as active ingredient a compound of formula:-  <FORM:1095184/A5-A6/1> in which each of R4 and R5 is a hydrogen atom or a C1-5 alkyl or aryl-substituted C1-5 alkyl group, or OR4 and OR5 are adjacent to each other and R4 and R5 together represent methylene; M is a C1-5 alkoxy, an aryl-substituted C1-5 alkoxy, an amino, or a hydroxy group; and n is 0, 1 or 2, or an acid-addition salt thereof, and, if desired, another anti-hypertensive agent.