<PAGE>
 
                                  Exhibit 5.1

                                                                Washington, D.C.
                            BUTLER & BINION, L.L.P.              (202) 466-6900

                                                                       ---
ATTORNEYS AT LAW                  Suite 1600                          Dallas
     ---                        1000 Louisiana                   (214) 220-3100
                           Houston, Texas 77002-5093                   ---
                                (713) 237-3111                     San Antonio
                                                                 (512) 227-2200


                                  May 8, 1997



Reliability Incorporated
16400 Park Row
Houston, Texas  77084

        Re:  Registration of 500,000 shares of Common Stock of Reliability
             Incorporated pursuant to the 1997 Stock Option Plan

Gentlemen:

        We have acted as legal counsel for Reliability Incorporated, a Texas
corporation ("Company"), in connection with the offer to certain of the
directors, officers and key employees of the Company and its subsidiaries of up
to 500,000 shares of the Company's common stock, no par value per share ("Common
Stock"), issuable pursuant to the Company's 1997 Stock Option Plan (the "Plan").

        We have made such inquiries and examined such documents as we have
considered necessary or appropriate for the purposes of giving the opinion
hereinafter set forth, including the examination of executed or conformed
counterparts, or copies certified or otherwise proved to our satisfaction of the
following:

                (i) the Articles of Incorporation, as amended, of the Company as
        filed with the Secretary of State of Texas;

                (ii)  the Bylaws of the Company as of the date of this opinion;

                (iii) the Company's Registration Statement on Form S-8, covering
        Common Stock issuable pursuant to the Plan, to be filed with the
        Securities and Exchange Commission on May 8, 1997 ("Registration
        Statement");

                (iv)  the Plan;

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Reliability Incorporated
May 8, 1997
Page 2

 
                (v)  such other documents, corporate records, certificates and
        instruments as we have deemed necessary or appropriate for the purpose
        of this opinion.

        We have assumed the genuineness and authenticity of all signatures on
all original documents, the authenticity of all documents submitted to us as
originals, the conformity to originals of all documents submitted to us as
copies and the due authorization, execution, delivery or recordation of all
documents where due authorization, execution or recordation are prerequisites
to the effectiveness thereof.

        Based upon the foregoing, and having regard for such legal
considerations as we deem relevant, we are of the opinion that:

        The Common Stock covered by the Registration Statement has been duly
authorized and when issued and sold in accordance with the Plan will be legally
issued, fully paid and nonassessable.

        We hereby consent to the filing of this opinion with the Securities and
Exchange Commission as an exhibit to the Registration Statement.


                                        Very truly yours,

                                        /s/ BUTLER & BINION, L.L.P.

                                        BUTLER & BINION, L.L.P.


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