The Collaborative International Dictionary of English v.0.48:

Inalterable \In*al"ter*a*ble\, a. [Pref. in- not + alterable:
   cf. F. inalt['e]rable.]
   Not alterable; incapable of being altered or changed;
   unalterable. -- In*al"ter*a*ble*ness, n. --
   In*al"ter*a*bly, adv.
   [1913 Webster]

WordNet (r) 3.0 (2006):

inalterable
    adj 1: not capable of being changed or altered; "unalterable
           resolve"; "an unalterable ground rule" [syn:
           unalterable, inalterable] [ant: alterable]

Moby Thesaurus II by Grady Ward, 1.0:

72 Moby Thesaurus words for "inalterable":
   assiduous, constant, continuing, diligent, dogged, enduring,
   faithful, fixed, immovable, immutable, indefatigable, indomitable,
   industrious, insistent, invariable, invincible, lasting, loyal,
   never-tiring, obstinate, patient, patient as Job, permanent,
   perseverant, persevering, persistent, persisting, pertinacious,
   plodding, plugging, preoccupied, rapt, relentless, resolute,
   sedulous, single-minded, sleepless, slogging, stable, steadfast,
   steady, stubborn, tenacious, tireless, unabating, unalterable,
   unchangeable, unconquerable, undaunted, undiscouraged, undrooping,
   unfailing, unfaltering, unflagging, unflinching, unintermitting,
   uninterrupted, unmodifiable, unmovable, unnodding, unrelaxing,
   unrelenting, unremitting, unsleeping, unswerving, untiring,
   unwavering, unwearied, unwearying, unwinking, utterly attentive,
   weariless